Do /metrics/find queries in parallel

Based on what was done to fetchData() in #1026.

`/metrics/find/` queries are used less often since #1010 went in, but the composer still uses them.
This makes browsing metrics through the composer much less painful :-)

The code has been very lightly tested (it is December 30th code after all), but it works on a 10 node cluster.

cc @bmhatfield