HDDS-6168. Add config to disable registering S3 Gateway metrics

[vyalamar]

Summary

• add config toggle to disable S3G metrics (ozone.s3g.metrics.enabled, default true)
• make S3GatewayMetrics.create idempotent and add JVM shutdown hook to unregister; return null when disabled
• regression test to assert create() is idempotent

Testing

• mvn -pl hadoop-ozone/s3gateway -am -DskipITs test (ran locally; completed via cached modules)

[vyalamar]

@adoroszlai, @ivanzlenko, and @Russole pls help review.

[adoroszlai]

Thanks @vyalamar for the patch.

[adoroszlai]

Let's cache the result in a static Boolean variable.

if (metricsEnabled == null) {
  metricsEnabled = conf.getBoolean(...);
}
if (!metricsEnabled) {
  return null;
}

[yandrey321]

isn't it supposed to be called once? in this case there is no need for a cache.

[adoroszlai]

Thanks @vyalamar for updating the patch.

[vyalamar]

@adoroszlai updated.

[yandrey321]

would it produce non-null instance after this call?

On another look, I doubt this patch fixes the problem reported in HDDS-6168. S3GatewayMetrics was only introduced in HDDS-6481, 2 months after HDDS-6168. So the memory issue must have been about other kinds of metrics (the ones standard across all server components).

[vyalamar]

@adoroszlai sure, let me go to older version of the code and try to reproduce?

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