This PR introduces the internal recordMetric function, which emits the gcp.client.request.duration metric.

It also introduces two exported APIs:

1. apierror.Message which returns the raw string (e.g. the "server error" part of an HTTP response that doesn't have an error.)
2. gax.ParseTelemetryErrorInfo takes a context and error and populates a struct with all the required error information for tracing, logging and metrics.

Subsequent PRs will hook this function into gax.Invokeand expand the recorded attributes with dynamic error statuses and transport metadata.

We would add this to invoke:

	if IsFeatureEnabled("METRICS") {
		start := time.Now()
		defer func() {
			recordMetric(ctx, settings, time.Since(start), err)
		}()
	}

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