Watcher Stats metrics parameters are mislabeled

[imotov]

While working testing HL REST client for Watcher Stats API I have noticed that the metirics parameters seem to be mislabeled:

GET /_xpack/watcher/stats/queued_watches

returns

{
...
      "current_watches": []
...
}

while

GET /_xpack/watcher/stats/pending_watches

returns

{
...
      "queued_watches": []
...
}

After discussions with @spinscale and @martijnvg, @spinscale suggested that we should

1. queued_watches metric should return queued_watches instead of current_watches and
2. pending_watches should be renamed into current_watches

This is a breaking change, but since nobody noticed reported this discrepancy in the last 2 years, its impact will probably be relatively minimal.

[elasticmachine]

Pinging @elastic/es-core-infra