Fix Prometheus metrics MIME type and bump prometheus-client to 0.21.1

[psrok1]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the current behaviour?

• /api/varz offers Prometheus metrics but sends them with generic application/octet-stream Content Type.
• Because of that, there are issues with compatibility with Prometheus v3 which is more strict concerning the Content-Type and doesn't automatically fallback to legacy protocol unless it's configured to do so (https://prometheus.io/docs/prometheus/latest/migration/).

What is the new behaviour?

• Set correct Content-Type for the response (got from prometheus_client.CONTENT_TYPE_LATEST)
• Bumped prometheus-client to latest. I initially thought that 0.20.0 updates the PrometheusText version to 1.0.0, but misread the changelog and the bump was for OpenMetricsText, so the format we're using stays on 0.0.4. Anyway, it's always a good opportunity to bump the version.