Exposition format: Termination marker for scrapes

Taken from the original issue https://github.com/prometheus/common/issues/14 :

To positively confirm the completion of a scrape (rather than only negatively detect an incomplete scrape by a connection error), a termination marker is needed.

Proposal:

If an exposition is meant to be terminated by a termination marker, it's content-type contains an additional parameter termination=marked. (It will still be possible to not use termination markers so that trivial one-line expositions are still possible by default.) Termination can be requested via the usual content-type negotiation, and should only be applied if requested.

With termination=marked, the following termination marker has to end the payload:

• A line # END\n for the text format.
• A uvarint-encoded value of MaxUint32 in case of protobuf. (Has a length of 5 bytes - protobufs of that size cannot be encoded anymore but the value is any waymeant to be larger than a protobuf can reasonably be. Some protobuf implementation limit the size of a proto message at 64MiB anyway.)

An exposition with termination=marked and no termination marker is considered incomplete, which should be treated the same as an ingestion interrupted by a connection error.

Start by reading the linked original issue in prometheus/common and the proposal here, focusing on content-type negotiation and the text and protobuf termination markers. No files, tests, or entry points are named; completion would require a maintainer decision on scope and where this protocol change belongs.