Add support for custom MR mapping

[psx95]

Description

Provides a configuration option for the metrics exporter allowing users to specify mappings for custom MonitoredResource type.

The mapping logic for the custom MR type is activated based on the presence of gcp.resource_type resource attribute in the underlying OpenTelemetry resource.

Sample usage with the proposed config changes

exporterOpts := []Option {
                WithProjectID(projectID),
		WithMonitoringClientOptions(clientOpts...),
		WithMetricDescriptorTypeFormatter(formatter),
		WithMonitoredResourceDescription("custom_mr", []string{"service_instance_id", "host_id", "location"}),
}

exporter, err := New(opts...)
... 
// If the OTel resource now has the Attribute label `gcp.resource_type` with value 
// `custom_mr` the exporter will attempt to map the OTel resource to the set MR type.

Testing

• Tested this newly added configuration with the existing metrics example.
• Used the configuration to override MR type to dataflow-job.
• The program returned valid error when all required labels were not present in the resource attributes.
• When all required labels were present, metric was successfully written against the correct MR type.

Configuration code for reference

// create the exporter with the Monitored Resource Description
exporter, err := mexporter.New(mexporter.WithProjectID(os.Getenv("GOOGLE_PROJECT_ID")), mexporter.WithMonitoredResourceDescription("dataflow_job", []string{"job_name", "project_id", "region"}))
	if err != nil {
		log.Fatalf("Failed to create exporter: %v", err)
	}

...

// Make sure the OTel resource has the `gcp.resoure_type` along with the required labels configured.
 res, err := resource.New(
		ctx,
		// Use the GCP resource detector to detect information about the GCP platform
		resource.WithDetectors(gcp.NewDetector()),
		// Keep the default detectors
		resource.WithTelemetrySDK(),
		// Add attributes from environment variables
		resource.WithFromEnv(),
		// Add your own custom attributes to identify your application
		resource.WithAttributes(
			semconv.ServiceNameKey.String("example-application"),
			attribute.KeyValue{Key: "gcp.resource_type", Value: attribute.StringValue("dataflow_job")},
			attribute.KeyValue{Key: "job_name", Value: attribute.StringValue("some-job")},
			attribute.KeyValue{Key: "region", Value: attribute.StringValue("us1")},
		),
	)

// Construct the exporter using the above config
provider := sdkmetric.NewMeterProvider(
		sdkmetric.WithReader(sdkmetric.NewPeriodicReader(exporter)),
		sdkmetric.WithResource(res),
	)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.92%. Comparing base (4caace7) to head (0f9bfad).
Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #854      +/-   ##
==========================================
+ Coverage   61.03%   62.92%   +1.88%     
==========================================
  Files          56       57       +1     
  Lines        5903     4955     -948     
==========================================
- Hits         3603     3118     -485     
+ Misses       2143     1677     -466     
- Partials      157      160       +3     

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