feat(v2): implement basic metric recording function

[westarle]

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)
		}()
	}

64a4698

[shollyman]

Do we need to define an epsilon error bound since we're testing floating point equality? We're probably okay since we're only testing a literal duration transformation, but if we find jitter is exposed from the otel layer it might be something to add later.

[westarle]

I think it's a good idea, I added it.

[quartzmo]

This PR copies/adapts logic from cloud.google.com/go/auth (googleapis/google-cloud-go#14133). Gax seems to me to be the better home for this logic. Can we export it in this PR and then update cloud.google.com/go/auth to use the Gax exports?

[quartzmo]

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

Can you provide a code snippet preview in the PR description of how you think Invoke/invoke will call recordMetric? That will help me with the review.

[quartzmo]

Nice, this is good practice.

[quartzmo]

As a pure parser that now returns a clean data struct from inputs, this is a big improvement over the original version in auth. Nice work!