Plugins: Make backend plugin metrics endpoints available with optional authentication (#46467)

* add new endpoint without auth+config

* add cfg check

* fit lint issue

* Add basic auth support

Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>

* WIP docs

* Update docs/sources/administration/view-server/internal-metrics.md

Co-authored-by: Dave Henderson <dhenderson@gmail.com>

* update instructions

Co-authored-by: Will Browne <will.browne@grafana.com>
Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>
Co-authored-by: Dave Henderson <dhenderson@gmail.com>

devenv/docker/blocks/prometheus/prometheus.yml

@@ -37,3 +37,8 @@ scrape_configs:
   - job_name: 'prometheus-random-data'
     static_configs:
       - targets: ['prometheus-random-data:8080']
+
+  # - job_name: 'grafana-test-datasource'
+  #   metrics_path: /metrics/plugins/grafana-test-datasource
+  #   static_configs:
+  #     - targets: ['host.docker.internal:3000']