Proxy exposes Prometheus protobuf metrics

[hawkw]

The proxy metrics endpoint (added in #540) should also expose metrics in the Prometheus Protocol Buffers format. Whether to serve text metrics or protobuf should be determined based on the request's content type, with text/plain requests receiving text metrics and application/vnd.google.protobuf; proto=io.prometheus.client.MetricFamily; encoding=delimited requests receiving protobuf metrics.

[hawkw]

According to the Prometheus documentation:

NOTE: Prometheus 2.0 removed support for the protocol-buffer format and only supports the text-based format.

Are we still interested in supporting the protocol buffer format? We might want to for non-Prometheus 2.0 installations, but removing support for this could potentially make our metrics implementation much more efficient (as prost requires a lot of things to be Vecs or Strings that could otherwise be non-allocating).