[java-bigtable] Replace the usage of BaseApiTracer and BaseApiTracerFactory with ApiTracer and ApiTracerFactory

As part of the OpenTelemetry metrics project, we made all methods in `ApiTracer` default in this [PR](https://github.com/googleapis/sdk-platform-java/pull/2403/files#diff-7c0438b5f6b77d8cf3f1bf3eb9c4550c8a47852bbf16c42a367797c48f3f584f). Since `BaseApiTracer` was originally introduced for Java 7 compatibility and we dropped Java 7 support now, we can replace the usage of `BaseApiTracer` and `BaseApiTracerFactory` with `ApiTracer` and `ApiTracerFactory`.
For example, make `BigtableTracer` implements `ApiTracer` instead of [extending from BaseApiTracer](https://github.com/googleapis/java-bigtable/blob/3f37d8da8b51a3fea56d1c462b9500ee2c244973/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/stub/metrics/BigtableTracer.java#L29).