The playbook exposes service metrics under a single endpoint
(`https://matrix.example.com/metrics/*`), controlled by
`matrix_metrics_exposure_enabled` and friends. synapse-usage-exporter was
not wired into this at all, so its metrics could only be scraped by the
integrated Prometheus and were unreachable for an external one.

The role had no metrics router whatsoever. Its only Traefik router (for
the `/report-usage-stats/push` ingestion endpoint) was gated on
`matrix_synapse_usage_exporter_container_labels_traefik_enabled`, which
defaulted to `matrix_synapse_usage_exporter_proxying_enabled`. Adding a
metrics router under that same switch would have meant that exposing
metrics also publishes the ingestion endpoint, so the master switch now
defaults to `true` and each router carries its own conditional.

Despite its generic name, `matrix_synapse_usage_exporter_proxying_enabled`
only ever concerned the endpoint that Synapse pushes usage statistics
**to**, never the metrics endpoint that Prometheus reads **from**. The
documentation now says so explicitly.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/5467

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>