2 small metrics fixes

Two fixes for metrics:

- add core.avg_response_time metric
- don't add worker 0 to total metrics

Average response time could be in worker.0 but I would need to hack for loop so it would get ugly and other total-* metrics are in core, so it seems better to keep them together. worker.0 tree contains few useless metrics right now (like worker.0.avg_response_time or worker.0.delta_requests), could I get rid of them?

Adding worker.0 to total metrics seems wrong, worker.0 since AFAIK worker.0 already contains total stats.
Also adding core metrics for worker.0 seems unneeded, they contain zeros all the time, so I've filtered it.

It should be 2 patches, but I need to also filter out worker.0 for avg_response_time so first patch would overlap second one.

Please review