[sdk-platform-java] Wrap expensive log statements in isLoggable check

String manipulation that occurs before invoking the log method:
* https://github.com/googleapis/sdk-platform-java/blob/7fc555c20c642e813ee60fc27e05966143afc924/gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/ChannelPool.java#L290-L301

And a method that performs relatively complex logic (env var checking, file reading) before choosing what to log:
* https://github.com/googleapis/sdk-platform-java/blob/7fc555c20c642e813ee60fc27e05966143afc924/gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java#L275
* The first check in this method could check the log level and exit early if no logging will be performed.

A quick search of other log uses in sdk-platform-java and google-cloud-java did not show any other substantial improvements.