add readme for otlp metric sample (#1005)

example/metric/otlpgrpc/README.md

@@ -0,0 +1,24 @@
+# OTLP Metric with Google Auth Example
+
+Run this sample to connect to an endpoint that is protected by GCP authentication.
+
+#### Prerequisites
+
+Get Google credentials on your machine:
+
+```sh
+gcloud auth application-default login
+```
+
+#### Run the Sample
+
+```sh
+# export necessary OTEL environment variables
+export PROJECT_ID=<project-id>
+export OTEL_EXPORTER_OTLP_ENDPOINT=<endpoint>
+export OTEL_RESOURCE_ATTRIBUTES="gcp.project_id=$PROJECT_ID,service.name=otlp-sample,service.instance.id=1"
+export OTEL_EXPORTER_OTLP_HEADERS=X-Goog-User-Project=$PROJECT_ID
+
+# from the samples/otlpmetric repository
+cd example/metric/otlpgrpc && go run .
+```