feat(Spanner): Add the built in metrics feature

[Hectorhammett]

Adds the Client built in metrics for Spanner.

b/406082122

[surbhigarg92]

"global" location is only a fallback , if customer application is not deployed on GCP. We need to detect Client location , if not available then fall back to GCP

Refer https://github.com/googleapis/java-spanner/blob/main/google-cloud-spanner/src/main/java/com/google/cloud/spanner/BuiltInMetricsProvider.java#L217

[surbhigarg92]

instance_id can not be blank as it is a monitored resource. If for some reason (should not be happening) the instance_id is not available use a default value like 'unknown' , else customer would see error

[Hectorhammett]

So in reality this should never be empty, but I have this empty string as a fallback.

Is it preferable to instead not record any data in the case that we are missing this piece of information?

[surbhigarg92]

I would recommend recording the data with "unknown" value, If we see some customers data with instance_id as "unknown" then we would know there is an issue. If we simply ignore the data then the logs will only be available with customers, and we will never get to know about the issue.

Eg: we were able to catch a edge case when we saw metrics data with "unknown" value in Java.

But this is not a hard requirement, will leave it upto you. If you feel this shall never happen, then you may simply ignore it.

[surbhigarg92]

We should not be hardcoding the status to "OK" . We should wait for RPC completion and record the actual status. This is what has been done for other languages