feat: add otlp metric sample with gcp auth (#371)

* feat: add otlp metric sample with gcp auth

* populate required resource attributes

Author: dashpole

samples/otlpmetric/README.md

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+### OTLP Export Sample with GCP Auth
+This example shows how to send metrics to an OTLP (OpenTelemetry Protocol) endpoint that is protected by GCP authentication. The sample showcases the metric export using gRPC.
+
+#### Installation
+Install the dependencies and libraries required to run the samples:
+
+```sh
+# Move to the sample repository
+cd samples/otlpmetric
+
+pip install -r requirements.txt
+```
+
+#### 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"
+
+# from the samples/otlpmetric repository
+python3 example.py
+```

samples/otlpmetric/example.py

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+#!/usr/bin/env python3
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import time
+
+import google.auth
+import google.auth.transport.grpc
+import google.auth.transport.requests
+import grpc
+from google.auth.transport.grpc import AuthMetadataPlugin
+from opentelemetry import metrics
+from opentelemetry.exporter.otlp.proto.grpc.metric_exporter import (
+    OTLPMetricExporter,
+)
+from opentelemetry.resourcedetector.gcp_resource_detector._detector import (
+    GoogleCloudResourceDetector,
+)
+from opentelemetry.sdk.resources import SERVICE_NAME, Resource, get_aggregated_resources
+from opentelemetry.sdk.metrics import MeterProvider
+from opentelemetry.sdk.metrics.export import PeriodicExportingMetricReader
+
+"""
+This is a sample script that exports OTLP metrics encoded as protobufs via gRPC. 
+"""
+
+credentials, project_id = google.auth.default()
+request = google.auth.transport.requests.Request()
+resource = get_aggregated_resources(
+    [GoogleCloudResourceDetector(raise_on_error=True)]
+)
+
+auth_metadata_plugin = AuthMetadataPlugin(
+    credentials=credentials, request=request
+)
+channel_creds = grpc.composite_channel_credentials(
+    grpc.ssl_channel_credentials(),
+    grpc.metadata_call_credentials(auth_metadata_plugin),
+)
+
+exporter = OTLPMetricExporter(credentials=channel_creds)
+reader = PeriodicExportingMetricReader(exporter)
+provider = MeterProvider(metric_readers=[reader],resource=resource)
+meter = provider.get_meter("gcp.otlp.sample")
+counter = meter.create_counter("sample.otlp.counter")
+
+
+def do_work():
+    counter.add(1)
+    # do some work that the 'counter' will track
+    print("doing some work...")
+
+
+def do_work_repeatedly():
+    try:
+        while True:
+            do_work()
+            time.sleep(1)
+    except KeyboardInterrupt:
+        print("\nKeyboard Interrupt: Stopping work.")
+
+
+do_work_repeatedly()

samples/otlpmetric/requirements.txt

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+# Dependencies require for trace export samples
+opentelemetry-api==1.24.0
+opentelemetry-sdk==1.24.0
+google-auth==2.18.1
+opentelemetry-exporter-otlp-proto-grpc==1.24.0
+grpcio==1.63.0
+opentelemetry-resourcedetector-gcp==1.8.0