feat: Add support for open telemetry

[vi3k6i5]

Add support for open telemetry tracing if open telemetry is installed.
Added the same as extra dependency.

Also added unit tests for the same.

Follow up PR to add tracing in schema with related checks in unit tests. #648

Fixes #633 🦕

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[c24t]

The way we're testing OT is kind of surprising, but that's largely OT's fault for being difficult to test. A few minor comments, but LGTM.

[larkee]

So this adds the trace_call method but does not add any calls. This means there is no tracing being added to the library. Is the intention for users to add trace calls themselves?

For the Spanner Client and the SQLAlchemy connector, we added tracing within the libraries. See here and here for more context.

[vi3k6i5]

So this adds the trace_call method but does not add any calls. This means there is no tracing being added to the library. Is the intention for users to add trace calls themselves?

For the Spanner Client and the SQLAlchemy connector, we added tracing within the libraries. See here and here for more context.

So that will have to be a follow up PR, since this PR was already crossing 300 lines, I didn't add it here. Updated the description to reflect the same info.