feat: Add the MetricsTracerFactory for client side metrics

[danieljbruce]

Summary:

This PR adds the MetricsTracerFactory which the rest of the client library will use to collect client side metrics to send to GCP backend for analysis. None of the methods will use MetricsTracerFactory yet so this PR doesn't change client library behaviour in any way. It is just in the stage of building infrastructure to support client side metrics.

Changes:

TODO

Next Steps:

1. Write some system tests using this new test infrastructure that logs metrics collected and ensure the metrics collected are what we would expect.
2. Add client side metrics to readRows calls
3. Add client side metrics to other calls