FINE logging of toString taking up a significant amount of CPU #2048

For instance this line https://github.com/googleapis/java-firestore/blob/main/google-cloud-firestore/src/main/java/com/google/cloud/firestore/Watch.java#L477.

It does a toString on the entire snapshot and then discards it. The common pattern when doing expensive logging like this is to check if the log level is enabled before doing the heavy toString