Prometheus Content-Type negotiation

[fischerman]

Is your feature request related to a problem? Please describe.
Scraping Prometheus metrics requires the query parameter ?format=prometheus. In a Kubernetes setup, scraping endpoints tend be discovered using pod annotations (e.g. prometheus.io/path), however an annotation for parameters is not very common (? is escaped properly by Prometheus, so parameters cannot be provided in the path).

Describe the solution you'd like
Prometheus does Content-Type negotiation (see here). When Vault sees the schema schema="prometheus/telemetry" it could serve Prometheus metrics without having to specify the format parameter.

Describe alternatives you've considered
None

Explain any additional use-cases
None