Add docs for updating Otel dependencies

docs/developer/updating-otel.md

@@ -0,0 +1,250 @@
+# Updating OpenTelemetry Collector dependencies
+
+The Agent depends on various OpenTelemetry (Otel) modules such as these:
+```
+github.com/open-telemetry/opentelemetry-collector-contrib/exporter/jaegerexporter v0.80.0
+github.com/open-telemetry/opentelemetry-collector-contrib/extension/sigv4authextension v0.80.0
+github.com/open-telemetry/opentelemetry-collector-contrib/processor/spanmetricsprocessor v0.80.0
+go.opentelemetry.io/collector v0.80.0
+go.opentelemetry.io/collector/component v0.80.0
+go.opentelemetry.io/otel v1.16.0
+go.opentelemetry.io/otel/metric v1.16.0
+go.opentelemetry.io/otel/sdk v1.16.0
+```
+
+The dependencies mostly come from these repositories:
+
+* [opentelemetry-collector](https://github.com/open-telemetry/opentelemetry-collector)
+* [opentelemetry-collector-contrib](https://github.com/open-telemetry/opentelemetry-collector-contrib)
+* [opentelemetry-go](https://github.com/open-telemetry/opentelemetry-go)
+
+Unfortunately, updating Otel dependencies is not straightforward:
+
+* Some of the modules in `opentelemetry-collector` come from a [grafana/opentelemetry-collector](https://github.com/grafana/opentelemetry-collector) fork. 
+  * This is mostly so that we can include metrics of Collector components with the metrics shown under the Agent's `/metrics` endpoint.
+* All Collector and Collector-Contrib dependencies are kept in sync on the same version.
+  * E.g. if we use `v0.80.0` of `go.opentelemetry.io/collector`, we also use `v0.80.0` of `spanmetricsprocessor`.
+  * This is in line with how the Collector itself imports dependencies.
+  * It helps us avoid bugs.
+  * It makes it easier to communicate to customers the version of Collector which we use in the Agent.
+  * Unfortunately, updating everything at once makes it tedious to check if any of our docs or code need updating due to changes in Collector components. A lot of these checks are manual - for example, cross checking the Otel config and Otel documentation between versions.
+  * There are some exceptions for modules which don't follow the same versioning. For example, `collector/pdata` is usually on a different version, like `v1.0.0-rcv0013`.
+
+## Updating walkthrough
+
+### Update the Grafana fork of Otel Collector
+
+1. Create a new release branch with a `-grafana` suffix under [grafana/opentelemetry-collector](https://github.com/grafana/opentelemetry-collector). For example, if porting branch `v0.81.0`, make a branch under the fork repo called `0.81-grafana`.
+2. Check which branch of the fork repo the Agent currently uses.
+3. Create a PR to cherry-pick the same changes to the new `-grafana` branch.
+
+### Update the Agent's dependencies
+
+1. Make sure we use the same version of Collector and Collector-Contrib for all relevant modules. For example, if we use version `v0.81.0` of Collector, we should also use version `v0.81.0` for all Contrib modules.
+2. Update the `replace` directives in the go.mod file to point to the latest commit of the forked release branch. Use a command like this:
+   ```
+   go mod edit -replace=go.opentelemetry.io/collector=github.com/grafana/opentelemetry-collector@asdf123jkl
+   ```
+   Repeat this for any other modules where a replacement is necessary.
+3. Note that sometimes Collector depends on packages with "rc" versions such as `v1.0.0-rcv0013`. This is ok, as long as the go.mod of Collector also references the same versions - for example, [pdata](https://github.com/open-telemetry/opentelemetry-collector/blob/v0.81.0/go.mod#L25) and [featuregate](https://github.com/open-telemetry/opentelemetry-collector/blob/v0.81.0/go.mod#L24).
+
+### Update otelcol Flow components
+
+1. Note which Otel components are in use by the Agent.
+   * Usually for every "otelcol" Flow component there is a corresponding Collector component.
+   * In some cases we don't use the corresponding Collector component:
+     * For example, [otelcol.receiver.prometheus](https://grafana.com/docs/agent/latest/flow/reference/components/otelcol.receiver.prometheus/) and [otelcol.exporter.prometheus](https://grafana.com/docs/agent/latest/flow/reference/components/otelcol.exporter.prometheus/).
+     * Those components usually have a note like this:
+       > NOTE: otelcol.exporter.prometheus is a custom component unrelated to the prometheus exporter from OpenTelemetry Collector.
+    * For example, the Otel component used by [otelcol.auth.sigv4](https://grafana.com/docs/agent/latest/flow/reference/components/otelcol.auth.sigv4/) is [sigv4auth](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/extension/sigv4authextension).
+2. Make a list of the components which have changed since the previously used version.
+   1. Go through the changelogs of both [Collector](https://github.com/open-telemetry/opentelemetry-collector/releases) and [Collector-Contrib](https://github.com/open-telemetry/opentelemetry-collector-contrib/releases).
+   2. If a component which is in use by the Agent has changed, note it down.
+3. For each Otel component which has changed, compare how they changed.
+   1. Compare the old and new version of Otel's documentation.
+   2. Compare the config.go file to see if new parameters were added.
+4. Update the Agent's code and documentation where needed.
+
+## Testing
+
+### Testing a tracing pipeline locally
+
+You can use the resources in the [Tempo repository](https://github.com/grafana/tempo/tree/main/example/docker-compose/agent) to create a local source of traces using k6. You can also start your own Tempo and Grafana instances.
+
+1. Comment out the "agent" and "prometheus" sections in the [docker-compose](https://github.com/grafana/tempo/blob/main/example/docker-compose/agent/docker-compose.yaml). We don't need this - instead, we will start our own locally built Agent.
+2. Change the "k6-tracing" endpoint to send traces on the localhost, outside of the Docker container.
+   * For example, use `ENDPOINT=host.docker.internal:4320`.
+   * Then our local Agent should be configured to accept traces on `0.0.0.0:4320`.
+3. Optionally, e.g. if you prefer Grafana Cloud, comment out the "tempo" and "grafana" sections of the docker-compose file.
+4. Add a second k6 instance if needed - for example, when testing a Static Agent which has 2 Traces instances.
+
+### Static mode
+
+The [tracing subsystem](https://grafana.com/docs/agent/latest/static/configuration/traces-config/) is the only part of Static mode which uses Otel. Try to test as many features of it using a config file like this one:
+
+<details>
+  <summary>Example Static config</summary>
+
+```
+server:
+  log_level: debug
+
+logs:
+  positions_directory: "/Users/ExampleUser/Desktop/otel_test/test_log_pos_dir"
+  configs:
+    - name: "grafanacloud-oteltest-logs"
+      clients:
+        - url: "https://logs-prod-008.grafana.net/loki/api/v1/push"
+          basic_auth:
+            username: "USERNAME"
+            password: "PASSWORD"
+
+traces:
+  configs:
+  - name: firstConfig
+    receivers:
+      otlp:
+        protocols:
+          grpc:
+            endpoint: "0.0.0.0:4320"
+    remote_write:
+      - endpoint: tempo-prod-06-prod-gb-south-0.grafana.net:443
+        basic_auth:
+          username: "USERNAME"
+          password: "PASSWORD"
+    batch:
+      timeout: 5s
+      send_batch_size: 100
+    automatic_logging:
+      backend: "logs_instance"
+      logs_instance_name: "grafanacloud-oteltest-logs"
+      roots: true
+    spanmetrics:
+      handler_endpoint: "localhost:8899"
+      namespace: "otel_test_"
+    tail_sampling:
+      policies:
+        [
+          {
+            name: test-policy-4,
+            type: probabilistic,
+            probabilistic: {sampling_percentage: 100}
+          },
+        ]
+    service_graphs:
+      enabled: true
+  - name: secondConfig
+    receivers:
+      otlp:
+        protocols:
+          grpc:
+            endpoint: "0.0.0.0:4321"
+    remote_write:
+      - endpoint: tempo-prod-06-prod-gb-south-0.grafana.net:443
+        basic_auth:
+          username: "USERNAME"
+          password: "PASSWORD"
+    batch:
+      timeout: 5s
+      send_batch_size: 100
+    tail_sampling:
+      policies:
+        [
+          {
+            name: test-policy-4,
+            type: probabilistic,
+            probabilistic: {sampling_percentage: 100}
+          },
+        ]
+    service_graphs:
+      enabled: true
+
+```
+
+</details>
+
+Run this file for two types of Agents - an upgraded one, and another one built using the codebase of the `main` branch. Check the following:
+
+* Open `localhost:12345/metrics` in your browser for both Agents.
+  * Are new metrics added? Mention them in the changelog.
+  * Are metrics missing? Did any metrics change names? If it's intended, mention them in the changelog and the upgrade guide.
+* Try opening `localhost:8888/metrics` in your browser for the new Agent. 8888 is the Collector's default port for exposing metrics. Make sure this page doesn't display anything - the Agent should use port `12345` instead.
+* Check the logs for errors or anything else that's suspicious.
+* Check Tempo to make sure the traces were received.
+* Check Loki to make sure the logs generated from traces got received.
+* Check `localhost:8899/metrics` to make sure the span metrics are being generated.
+
+### Flow mode
+
+The "otelcol" [components](https://grafana.com/docs/agent/latest/flow/reference/components/) are the only part of Flow mode which uses Otel. Try to test as many of them as possible using a config file like this one:
+
+<details>
+  <summary>Example Flow config</summary>
+
+```
+otelcol.receiver.otlp "default" {
+    grpc {
+        endpoint = "0.0.0.0:4320"
+    }
+
+    output {
+        traces  = [otelcol.processor.batch.default.input]
+    }
+}
+
+otelcol.processor.batch "default" {
+    timeout = "5s"
+    send_batch_size = 100
+
+    output {
+        traces  = [otelcol.processor.tail_sampling.default.input]
+    }
+}
+
+otelcol.processor.tail_sampling "default" {
+  decision_wait               = "5s"
+  num_traces                  = 50000
+  expected_new_traces_per_sec = 0
+
+  policy {
+    name = "test-policy-1"
+    type = "probabilistic"
+
+    probabilistic {
+      sampling_percentage = 10
+    }
+  }
+
+  policy {
+    name = "test-policy-2"
+    type = "status_code"
+
+    status_code {
+      status_codes = ["ERROR"]
+    }
+  }
+
+  output {
+    traces = [otelcol.exporter.otlp.default.input]
+  }
+}
+
+otelcol.exporter.otlp "default" {
+    client {
+        endpoint = "localhost:4317"
+        tls {
+            insecure = true
+        }
+    }
+}
+```
+
+</details>
+
+Run this file for two types of Agents - an upgraded one, and another one built using the codebase of the `main` branch. Check the following:
+
+* Open `localhost:12345/metrics` in your browser for both Agents.
+  * Are new metrics added? Mention them in the changelog.
+  * Are metrics missing? Did any metrics change names? If it's intended, mention them in the changelog and the upgrade guide.
+* Check the logs for errors or anything else that's suspicious.
+* Check Tempo to make sure the traces were received.