feat(fluentd): allow fluentd_thread label to be configurable

[taharah]

Signed-off-by: Trevor Wood Trevor.G.Wood@gmail.com

What this PR does / why we need it:

This PR allows the fluentd_thread label included when multi-threading flushing is enabled to be configurable. The label will be included by default in order to preserve backwards compatibility.

Which issue(s) this PR fixes:
Fixes #5336

Special notes for your reviewer:

Checklist

• Documentation added
• Tests updated
• Is this an important fix or new feature? Add an entry in the CHANGELOG.md.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md -- N/A

[CLAassistant]

All committers have signed the CLA.

[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%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%

[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%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%

[owen-d]

The idea presented here looks good to me, but I'm not ruby expert :( . Now that loki accepts out of order data by default, this label isn't necessary in most scenarios.

cc @cyriltovena hopefully can give you an approval as well.