Make monitoring able to use prometheus push gateway

[ericvaandering]

Description

From a discussion with @rcarpa and @dchristidis :

@ericvaandering: What i would really like to be able to do with the current metrics is get access to the registry object so that these things could be pushed. I wonder if we could take a while to discuss this.

@rcarpa:
I don't think there is really a point to discuss it. Most infrastructure to use a custom registry is already there in rucio.core.monitor. Just add an optional registry argument in the constructor of MetricManager (if not set, defaults to REGISTRY) and pass it through to created metrics. You can also add a method to MetricManager to push to a pushgateway. Seems like a good idea. what do you think ?

@ericvaandering:
That's the thing. I don't see a registry in MetricManager, just in MulitMetric. Am I missing something? Otherwise, that plan is exactly what I would like.

@rcarpa:
There is none; but it can be (very easily) added if we want it there 😉

Motivation

No response

Change

No response