Stop the filelog-storage overlay disabling cloud auth

The collector exits at startup whenever the Grafana Cloud stack is brought up:

  cannot start pipelines: failed to start "otlphttp/grafanacloud" exporter:
  failed to resolve authenticator "basicauth/grafanacloud": authenticator not found

The developer stack passes otel-collector-filestorage.yaml as a second --config to
persist filelog read offsets. The collector replaces lists on merge rather than
appending them, which that file notes, and it repeats the BASE config's
service.extensions -- [health_check, file_storage/filelog]. Layer the cloud
overlay on top and the base config is swapped for the cloud one, whose list also
carries basicauth/grafanacloud; the storage overlay still comes last, so the
authenticator is dropped and the exporter that needs it cannot start.

Nothing degrades: the collector refuses to start at all, so a host brought up this
way exports no telemetry anywhere, local or cloud.

Adds a cloud variant of the overlay naming all three extensions, mounted by
docker-compose.grafanacloud.yaml over the base one so the command line is
unchanged. The base variant stays as it is for stacks that do not use the cloud
exporter.

docker/telemetry/docker-compose.grafanacloud.yaml

@@ -22,6 +22,10 @@ services:
       # Mount the Grafana Cloud collector config over the default path
       # (replaces local-only config for this run; file on disk untouched).
       - ./otel-collector-config.grafanacloud.yaml:/etc/otel-collector-config.yaml:ro
+      # Override the base filelog-storage overlay with the cloud variant. The
+      # collector replaces service.extensions on merge, and the base variant
+      # omits basicauth/grafanacloud, which stops the cloud exporter starting.
+      - ./otel-collector-filestorage.grafanacloud.yaml:/etc/otel-collector-filestorage.yaml:ro
     # Secrets from the env file, injected as container env and
     # resolved by the collector via ${env:...}.
     env_file:

docker/telemetry/otel-collector-filestorage.grafanacloud.yaml

@@ -0,0 +1,26 @@
+# Filelog offset persistence for the Grafana Cloud stack.
+#
+# Same purpose as otel-collector-filestorage.yaml, and needed for the same
+# reason, but it cannot be reused as-is. The collector replaces lists on merge
+# rather than appending, so whichever config is passed last decides
+# service.extensions outright. The base variant repeats the base config's list,
+# [health_check, file_storage/filelog], which drops basicauth/grafanacloud and
+# leaves the cloud exporter unable to resolve its authenticator -- the collector
+# then exits at startup with "authenticator not found" rather than degrading.
+#
+# Applied by docker-compose.grafanacloud.yaml, which mounts it over the base
+# variant so the command line stays unchanged.
+
+extensions:
+  file_storage/filelog:
+    directory: /var/lib/otelcol/file_storage
+    create_directory: true
+
+receivers:
+  filelog:
+    storage: file_storage/filelog
+
+# Must name every extension the cloud config needs, not just this file's own:
+# lists are replaced, so anything omitted here is omitted from the collector.
+service:
+  extensions: [health_check, file_storage/filelog, basicauth/grafanacloud]