[ADMIN_API] Adjusting worker messages

Changing PING back to STATS and STATS to METRICS to simplify deployments

Change-Id: I959f968a9a298c66b7e4fbfa6b6818acd95d8718
[WORKER] Change PING -> STATS, STATS -> METRICS

Gearman message actions need to change to maintain backward
compatibility. PING will revert back to STATS, and METRICS
will replace the STATS message.

Worker message documentation is also updated.

Change-Id: I1aa7478abcf29ffe5dbd512b2175dc5633548064
Merge "[WORKER] Change PING -> STATS, STATS -> METRICS"