Add channel latency to veneer

In bigtable-client-core, channel latency is reported from `ChannelPool` and each channel is assigned an ID at [channel creation time](https://github.com/googleapis/java-bigtable-hbase/blob/master/bigtable-client-core-parent/bigtable-client-core/src/main/java/com/google/cloud/bigtable/grpc/io/ChannelPool.java#L115).

After migrating to veneer, we could migrate this metric by adding an interceptor. However, with this implementation, channel id will be updated every hour if channel priming is enabled which could lead to high cardinality. There isn't an easy way to keep the same id for each channel without making some changes in gax.

Since `channels.channel$id.rpc.latency` is a trace level metric, this will not be implemented in the alpha release. We could add it back if necessary in the future.