Parallize label queries to both stores

[simonswine]

What this PR does / why we need it:

This executes label queries against ingesters and store in parallel, rather than sequentially.

Checklist
- [ ] Documentation added

• Tests updated
• Add an entry in the CHANGELOG.md about the changes.

[trevorwhitney]

LGTM, one small nit

[cyriltovena]

LGTM

[trevorwhitney]

thanks for that change, let's get this merged!

[stale]

Hi! This issue has been automatically marked as stale because it has not had any
activity in the past 30 days.

We use a stalebot among other tools to help manage the state of issues in this project.
A stalebot can be very useful in closing issues in a number of cases; the most common
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[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0.3%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%