fix(google-health): annotate an ended backfill in one fixed shape

The ended note carried the whole map of ended collections, dates and all,
which duplicated the slice report and gave dashboards no stable shape. It
now names the collection that just ended, its type and reason, whether the
end is final, and counts the ends so far with their types. It stays meta:
the job names the event's action once the slice is over, which would
replace an action set here.

docs/integrations/google-health.md

@@ -265,7 +265,10 @@ reading:
   nothing left but a data type waiting for its second attempt and queued the
   next slice for when it is due (`delaySeconds`).
 - `meta["googleHealth.backfill.ended"]` appears on the slice where a data
-  type ended short, with the same entries. A walk stopped for lack of
+  type ended short: the `collection`, its measurement `type`, the `reason`,
+  whether the end is `final`, how many data types have ended short so far
+  (`endedCount`) and of those for good (`finalCount`), and their `types`.
+  The full entries are in the slice report's `ended`. A walk stopped for lack of
   progress also leaves a warning:
   `fetchHeartRate: 20 pages in a row reached no further back than <instant>`.
 - `meta["googleHealth.backfill.edgeFilterUnsupported"]` appears on the slice