Fix incorrect Content-Type on /metrics endpoint

[fishi0x01]

What

common/status_app.py's /metrics handler returned generate_latest() bytes via web.Response(body=...) without an explicit content_type, so aiohttp defaulted the response's Content-Type header to application/octet-stream.

Why

Legacy Prometheus (2.x) parses the response body as the classic exposition text format regardless of Content-Type, so this went unnoticed. Prometheus 3.x is stricter and rejects ambiguous Content-Type outright unless a fallback_scrape_protocol is configured on the scrape side. App-SRE's newer Cluster Observability Operator stack runs Prometheus 3.x, so scrapes against listener, taskomatic, grouper, notificator, evaluator-upload, and evaluator-recalc (all of which use this shared status_app module) were failing with:

received unsupported Content-Type "application/octet-stream" and no fallback_scrape_protocol specified for target

while working fine under the legacy stack.

Fix

Explicitly set the Content-Type header to CONTENT_TYPE_LATEST, matching prometheus_client's own documented usage pattern (see exposition.py).

Ref: APPSRE-13041

Summary by Sourcery

Bug Fixes:

• Set the /metrics HTTP response Content-Type to Prometheus's CONTENT_TYPE_LATEST instead of relying on the aiohttp default.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Sets the Prometheus /metrics endpoint to return the correct text-based metrics Content-Type instead of the default application/octet-stream, by importing prometheus_client.CONTENT_TYPE_LATEST and applying it explicitly on the aiohttp response.

Sequence diagram for updated /metrics Prometheus scrape response

sequenceDiagram
  actor Prometheus
  participant StatusApp
  participant prometheus_client
  participant aiohttp_web as web

  Prometheus->>StatusApp: GET /metrics
  StatusApp->>StatusApp: metrics(request)
  StatusApp->>prometheus_client: generate_latest()
  prometheus_client-->>StatusApp: metrics_bytes
  StatusApp->>aiohttp_web: Response(body=metrics_bytes, headers={Content-Type: CONTENT_TYPE_LATEST})
  aiohttp_web-->>Prometheus: 200 OK Content-Type=CONTENT_TYPE_LATEST

Loading

File-Level Changes

Change	Details	Files
Ensure the /metrics endpoint responds with Prometheus' latest text exposition content type rather than aiohttp's default binary content type.	Import CONTENT_TYPE_LATEST from prometheus_client alongside existing metric utilities. Update the metrics() handler to pass headers={"Content-Type": CONTENT_TYPE_LATEST} to web.Response while still using generate_latest() for the body.	common/status_app.py

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