spanner: add oc tests for session pool metrics.

[hengfengli]

The tests verify the correctness of outputting session-pool metrics.

[hengfengli]

@olavloite Thanks for reviewing this CL on Gerrit. I have moved it here and will fix all your comments.

[hengfengli]

@olavloite

Patch Set #4, Line 36: t.Skip("TestExporter is not thread safe")

Not really related to this change, but I noticed it now: This problem was as far as I can remember fixed in https://code-review.googlesource.com/c/gocloud/+/54970, so I guess we can also enable this test as well.

Done.

Patch Set #4, Line 71: for _, test := range []struct {
Could this test be parallel?

No, I don't think so because it needs to register views a global view.

Patch Set #4, Line 214: server.TestSpanner.PutExecutionTime(stestutil.MethodGetSession,

As far as I can tell, GetSession will not be called by the code that follows in this test case. The client.Single().ReadRow(..) statement will create a session, but it will not call GetSession.

The get_session_timeouts statistic relates to the number of times the sessionPool.getReadSession(..) method will time out, but that method will never call the server method GetSession. Instead it will either wait for a session to come available when another process returns a session, or it will call (Batch)CreateSession(s) to create a new session. So in this case I would think that it would make more sense to set the execution time on one of those methods.

I may copy this from an existing test somewhere. I have deleted it.

[olavloite]

While it is highly likely that this is true, I'm a little bit afraid that this check might turn out to be flaky. The test uses a context with a timeout of 1ms, which means that it is highly likely to timeout, but there is no real guarantee that it will happen while waiting for a session. I think that adding a simulated execution time of 2ms for the BatchCreateSessions on the mock server would guarantee it.

[hengfengli]

Thanks for the suggestion. Done.

[olavloite]

Is this intentional?

[hengfengli]

Yes. I made some changes in https://github.com/googleapis/google-cloud-go/blob/master/internal/testutil/trace.go, which are included in v.0.58.0.

[olavloite]

LGTM once the merge conflicts are solved.