[sdk-platform-java] Metrics related to gax

## Question

This is a general question regarding how to get metrics around gax. (Or, maybe this is a feature request?)

Recently we realized that gax (transitive dependencies introduced by `java-bigtable` client library) tends to create a fairly large thread pool of many system threads to handle IO with Bigtable server. These threads are named with prefix `Gax-` so we think it is probably related to this `gax-java` library according to [this line](https://github.com/googleapis/gax-java/blob/1c4d535917cb1642f8da38a2a2c23ada4c15f394/gax/src/main/java/com/google/api/gax/core/InstantiatingExecutorProvider.java#L52).

We're not sure if the size of the thread pool is the culprit of causing regression of our service performance when the traffic goes high. Thus we'd like gain some observability into the gax threads first. However, I cannot find a good way to export the metrics such as "the number of gax threads" from time to time. For example, an option to register this metric to prometheus-metric client would be great.

We also noticed that Gax has a [channel pool resizing behavior](https://github.com/googleapis/gax-java/blob/fff2babaf2620686c2e0be1b6d338ac088248cf6/gax-grpc/src/main/java/com/google/api/gax/grpc/ChannelPool.java#L112-L125) over the time. It would also be nice to get metrics of channel pool size over the time and watch how the pool size get resized under different traffic patterns.