feat(bigtable): add client startup time metrics.

[sushanb]

TODO: on adding the transport type as we need to make Prime() rpc blocking.

[gemini-code-assist]

Summary of Changes

Hello @sushanb, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the Bigtable Go client by adding client startup time metrics. It measures the duration from the initiation of client creation to the full initialization of the connection pool, providing valuable insights into the client's readiness performance. The implementation uses OpenTelemetry for metric collection, with a planned future enhancement to accurately determine and record the transport type.

Highlights

• Client Startup Time Metrics: Introduced a new metric to measure the total time taken for the Bigtable client to complete its initialization logic within NewClientWithConfig.
• OpenTelemetry Integration: The startup time is recorded as a Float64Histogram using OpenTelemetry, with predefined bucket boundaries for latency analysis.
• Function Signature Update: The NewBigtableChannelPool function now accepts a startupTimer argument to facilitate the measurement of client startup duration.
• Test Coverage: Existing tests for NewBigtableChannelPool have been updated to pass the new startupTimer argument, and a dedicated test TestRecordClientStartUp has been added to verify the metric recording.

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[gemini-code-assist]

Code Review

This pull request adds a new metric to track client startup time, which is a valuable addition for monitoring. The implementation is mostly correct, but I've identified a few areas for improvement regarding code clarity, robustness, and testing. My feedback includes fixing a typo in a variable name, improving the metric recording logic to be more robust, and enhancing the corresponding unit tests.

[nimf]

The only thing I don't like in this PR is adding the new clientCreationTimestamp argument to the NewBigtableChannelPool function. It already has 5 arguments. We don't really need this argument because we can get the time instant inside the NewBigtableChannelPool. But we "need" this to test the client startup metric, so I was thinking how can we do this in another way.

We provide the dial function as one of the arguments. What if for that test we add time.Sleep inside the dialFunc we create in the test? As our test runs with synctest the test won't acutally sleep but rather fast-forward time.

On the other hand this is internal API, so we can leave it as is and make this change later. Up to you.

[sushanb]

The only thing I don't like in this PR is adding the new clientCreationTimestamp argument to the NewBigtableChannelPool function. It already has 5 arguments. We don't really need this argument because we can get the time instant inside the NewBigtableChannelPool. But we "need" this to test the client startup metric, so I was thinking how can we do this in another way.

We provide the dial function as one of the arguments. What if for that test we add time.Sleep inside the dialFunc we create in the test? As our test runs with synctest the test won't acutally sleep but rather fast-forward time.

On the other hand this is internal API, so we can leave it as is and make this change later. Up to you.

i agree as the most time intensive part is in NewBigtableChannelPool. Semantically, we want to measure the time when customer intiialize NewClientWithConfig to when client ready so that;s why we add the clientCreationTimestamp

[bhshkh]

@sushanb please do not merge PRs without first updating the branch.

#13552