InstrumentHandler should expose a gauge "requests in flight"

[neelance]

It would be great if InstrumentHandler could not only track completed requests by http_requests_total, but also have a metric of how many requests were received, e.g. http_requests_received. Then one could calculate http_requests_received - http_requests_total to get the number of concurrent requests that have not been processed yet.

[brian-brazil]

That'd be better done as a gauge. Direct arithmetic on counters can break in a number of fun ways such as resets and race conditions, and is to be avoided.

[neelance]

Okay, whatever you think is best. :-)

[beorn7]

Changed the title accordingly.

[beorn7]

Since InstrumentHandler is deprecated, this will be implemented as a middleware in the new promhttp package.