This PR revives #923, which was de-priotirized to work on the sync client. This PR brings it back, working with both async and sync. It also adds a grpc interceptor, as an improved way to capture metadata across both clients

TODO:

• blocked on https://github.com/googleapis/python-bigtable/pull/1174/files
• add tests for metadata capture during failed rpcs
• e2e tests for all metric collection?
• check for gaps in tests

Design

The main architecture looks like this:

Most of the work is done by the ActiveOperationMetric class, which is instantiated with each rpc call, and updated through the lifecycle of the call. When the rpc is complete, it will call on_operation_complete and on_attempt_complete on the MetricsHandler, which can then log the completed data into OpenTelemetry (or theoretically, other locations if needed)

Note that there are separate classes for active vs completed metrics (ActiveOperationMetric, ActiveAttemptMetric, CompletedOperationMetric, CompletedAttemptMetric). This is so that we can keep fields mutable and optional while the request is ongoing, but pass down static immutable copies once the attempt is completed and no new data is coming