Allow for pushing metrics to servers

[ericvaandering]

@rcarpa @bari12

Here's the push method and the specification/retrieval of the registry.

I also have a new version of the context manager originally defined here: https://github.com/rucio/probes/blob/7b8ba2d345dd9d1795d44f3fc4222183c62a89f8/common/utils/common.py#L39

Which lets you do something like

        with PrometheusPusher() as manager:
            (manager.gauge(name='statsd_name.{rse}.{country}.{rse_type}.{source}',
                           documentation='Space used at an RSE from various sources')
                    .labels(rse=rse['rse'], country=country, rse_type=rse_type, source=source)
                    .set(usage['used']))

And all the metrics you created in there get pushed at the end. Would you prefer I keep that in probes or put this in monitor.py too?