Add support for remote writing from one Prometheus to another

[simskij]

Enhancement Proposal

We need to support use cases where users want to tier their Prometheus deployments to avoid excessive pressure. By adding send-remote-write to Prometheus, and remote writing whatever we receive, we've effectively accomplished this.

[beliaev-maksim]

Note: reopen and append #411 when receive customer request with requirements.
For discussion around the feature please see #376

[PietroPasotti]

it was added at some point (#376) but it was later removed (when?, by whom? why?)
does @simskij remember? Or maybe @beliaev-maksim?

[beliaev-maksim]

@PietroPasotti implementation is ready (just reopen pr of needed) .

Rejected because back in time was not clear if alert rules should be forwarded as well or not