[Helm] Create metricsInstance only if available

[agologan]

What this PR does / why we need it:
If you're using prometheus operator and not Grafana Agent a metricsInstance shouldn't be auto-created.

Which issue(s) this PR fixes:
#7529

Special notes for your reviewer:
I would also propose moving the definition at the same level with serviceMonitor and adding .enabled i.e.:

  serviceMonitor:
    enabled: true
    ...
  metricsInstance:
    enabled: true
    annotations: {}
    labels: {}
    remoteWrite: null

This of course would be a breaking change.
L.E. Just noticed this other PR: #7525

Checklist

• Reviewed the CONTRIBUTING.md guide
• Documentation added
• Tests updated
• CHANGELOG.md updated
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md

[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%

[trevorwhitney]

I'd like to see a combination of this and #7525. I like where the enabled flag was added in #7525 as I feel it's a bit less disruptive, but I also like you capabilities check. Would you mind refactoring once #7525 is in to add your check to that same conditional where the new enabled is checked?

[agologan]

@trevorwhitney updated.
While I could've just written this big and, used the same pattern present in other files to be a bit more consistent.

[grafanabot]

./tools/diff_coverage.sh ../loki-target-branch/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%

[trevorwhitney]

lgtm, looks good, thanks!