More summary tests. WIP.

Change-Id: I8554733eb2c910857e9c6e2cdc8db4c20d19d886
Rename BenchmarkPrometheus... -> Benchmark...

Change-Id: I242cbb8bfba5a28ce43048366c6e940a386ea1de
Recycle mergedAllStreams.

Change-Id: I290e4236bfba55c7853148c28f803baea2339327