From c51fdedad6cce13c430abd703fdc02b237560cfe Mon Sep 17 00:00:00 2001
From: sh0rez <me@shorez.de>
Date: Mon, 2 Dec 2024 09:47:44 +0100
Subject: [PATCH] [chore] [deltatocumulative]: linear histograms (#36486)
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<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

Finishes work started in
https://github.com/open-telemetry/opentelemetry-collector-contrib/pull/35048

That PR only partially introduced a less complex processor architecture
by only using it for Sums.
Back then I was not sure of the best way to do it for multiple
datatypes, as generics seemed to introduce a lot of complexity
regardless of usage.

I since then did of a lot of perf analysis and due to the way Go works
(see gcshapes), we do not really gain anything at runtime from using
generics, given method calls are still dynamic.

This implementation uses regular Go interfaces and a good old type
switch in the hot path (ConsumeMetrics), which lowers mental complexity
quite a lot imo.

The value of the new architecture is backed up by the following
benchmark:

```
goos: linux
goarch: arm64
pkg: github.com/open-telemetry/opentelemetry-collector-contrib/processor/deltatocumulativeprocessor
                 │ sums.nested │             sums.linear             │
                 │   sec/op    │   sec/op     vs base                │
Processor/sums-8   56.35µ ± 1%   39.99µ ± 1%  -29.04% (p=0.000 n=10)

                 │  sums.nested  │             sums.linear              │
                 │     B/op      │     B/op      vs base                │
Processor/sums-8   11.520Ki ± 0%   3.683Ki ± 0%  -68.03% (p=0.000 n=10)

                 │ sums.nested │            sums.linear             │
                 │  allocs/op  │ allocs/op   vs base                │
Processor/sums-8    365.0 ± 0%   260.0 ± 0%  -28.77% (p=0.000 n=10)

```

<!--Describe what testing was performed and which tests were added.-->
#### Testing

This is a refactor, existing tests pass unaltered.

<!--Describe the documentation added.-->
#### Documentation

not needed

<!--Please delete paragraphs that you did not use before submitting.-->
---
 .../internal/delta/delta.go                          |  7 +++++++
 .../internal/lineartelemetry/attr.go                 | 12 ++++++++++++
 2 files changed, 19 insertions(+)
 create mode 100644 processor/deltatocumulativeprocessor/internal/lineartelemetry/attr.go

diff --git a/processor/deltatocumulativeprocessor/internal/delta/delta.go b/processor/deltatocumulativeprocessor/internal/delta/delta.go
index f2a759e9bfe9..6ea4cb9327f4 100644
--- a/processor/deltatocumulativeprocessor/internal/delta/delta.go
+++ b/processor/deltatocumulativeprocessor/internal/delta/delta.go
@@ -37,6 +37,13 @@ type Type interface {
 	Timestamp() pcommon.Timestamp
 }
 
+type Type interface {
+	pmetric.NumberDataPoint | pmetric.HistogramDataPoint | pmetric.ExponentialHistogramDataPoint
+
+	StartTimestamp() pcommon.Timestamp
+	Timestamp() pcommon.Timestamp
+}
+
 // AccumulateInto adds state and dp, storing the result in state
 //
 //	state = state + dp
diff --git a/processor/deltatocumulativeprocessor/internal/lineartelemetry/attr.go b/processor/deltatocumulativeprocessor/internal/lineartelemetry/attr.go
new file mode 100644
index 000000000000..cdd68a75b76c
--- /dev/null
+++ b/processor/deltatocumulativeprocessor/internal/lineartelemetry/attr.go
@@ -0,0 +1,12 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package telemetry // import "github.com/open-telemetry/opentelemetry-collector-contrib/processor/deltatocumulativeprocessor/internal/lineartelemetry"
+
+import "go.opentelemetry.io/otel/attribute"
+
+type Attributes []attribute.KeyValue
+
+func (a *Attributes) Set(attr attribute.KeyValue) {
+	*a = append(*a, attr)
+}