Fix race by properly locking access to scrape pools. Use separate mutex for information needed by UI so that UI isn't blocked when targets are being updated.
The semantics of honor_labels are that if a target exposes
and empty label it will override the target labels. This PR
fixes that by once again distinguishing between empty labels
and missing labels in this one use case.
Beyond that empty labels should be pruned and not added to storage,
which this also fixes.
Fixes#3841
