promtail: Support set tenant id from labels

[jkroepke]

What this PR does / why we need it:
Currently, it's not possible to set the tenant id based from label gathered by kubernetes_sd_configs. Configs like

scrape_configs:
  - job_name: kubernetes-pods-name
    
    kubernetes_sd_configs:
      - role: pod
    
    relabel_configs:
      - action: replace
        source_labels:
          - __meta_kubernetes_namespace
        target_label: namespace
    
    pipeline_stages:
    - match:
        selector: '{namespace=".+"}'
        stages:
          - tenant:
              source: "namespace"

won't work here.

This PR introduce a new option for the tenant stage to use the label map as source.

Since the tenant stage only sets an internal label, I tried to use

    relabel_configs:
      - action: replace
        source_labels:
          - __meta_kubernetes_namespace
        target_label: __tenant_id__

but it does not work, because all labels with __ are dropped by promtail.

Which issue(s) this PR fixes:
Fixes #

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

Related to #259 #701

[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%

[CLAassistant]

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

[slim-bean]

Looks great, thanks for this!