Clarify metric names related to batches/reports/observations.

Specifically use `observations` instead of `measures` in metric names,
which is our canonical name these days, despite some internal tasks
and tables still referring to the measures term.

Add new `items.uploaded.batch_size` pseudo-timer to track how many
reports per batch we get in the min/avg/max case and add a new
`items.uploaded.reports` counter. Finally fixed the
`items.uploaded.batches` counter to actually do what it says and count
incoming batches, so this metric is equal to the combined number of
successful API requests to the submit and geosubmit API's.
Add per API key metrics for batches, reports and observations, refs #368.