[metrics] Rename datum IDs

The new names make it clearer which ones are counters or histograms,
and what the units are for each one.

Bug: 170149255
Test: m
Merged-In: Ia7477bd84111fc42108ddaa20b1e50c6ceaeed71
Change-Id: Ia7477bd84111fc42108ddaa20b1e50c6ceaeed71
1 file changed