`/metrics` endpoint should set `Content-Type: text/plain; version=0.0.4` (Prometheus exposition format)

[jantman]

Background

The metrics endpoint (metrics.py) currently returns the Prometheus exposition body with:

Response(output, mimetype="text/plain; charset=utf-8")

That Content-Type has no version=0.0.4. Prometheus v2 tolerated this (it silently fell back to the 0.0.4 text format), but Prometheus v3 fails the scrape on a missing/unknown format version.

This surfaced during the DecaturMakers monitoring stack's Prometheus v2 → v3 upgrade. As a stopgap, the kiosk scrape job in dm-puppet now sets fallback_scrape_protocol: PrometheusText0.0.4, but the proper fix is to emit a correct Content-Type here so that shim can be removed.

The fix

Use prometheus_client's canonical content type instead of a hand-written one:

from prometheus_client import CONTENT_TYPE_LATEST, generate_latest
# ...
return Response(generate_latest(registry), mimetype=CONTENT_TYPE_LATEST)

CONTENT_TYPE_LATEST is text/plain; version=0.0.4; charset=utf-8.

Done when

• /metrics responds with Content-Type: text/plain; version=0.0.4; charset=utf-8
• New image released and the dmpuppet::internals::kiosk_show tag bumped in dm-puppet
• The fallback_scrape_protocol line is removed from the kiosk job in dm-puppet's prometheus.yml, and the kiosk target still scrapes on Prometheus v3

References

• dm-puppet: docs/features/completed/shared-infra_0.10.0_upgrade.md — finding Fix slideshow item creation and editing #2 / follow-up FU-1
• Sibling issue (ESP32 exporters): DecaturMakers/dm-puppet#33