allow overriding the remote read path

[BertHartm]

m3 uses a different path than the one that was hardcoded into promxy (ref: https://m3db.github.io/m3/coordinator/api/remote/#remote-read), so this allows setting the path in the config by server group.

This seems like it would potentially make path_prefix removable. I didn't attempt to remove for the sake of backwards compatibility.

[jacksontj]

We'll want to keep the path_prefix option as it applies to all the URLs downstream (remote_read, query, query_range, etc.) -- the use-case for that is when the promAPI is behind some proxy (somewhat common for those with lots of firewalls).

That all being said, this seems like a fine feature -- mind rebasing against latest master? Once thats done I can merge it in :)

[BertHartm]

Sounds good, just rebased.

Thanks

[jacksontj]

Awesome, thanks for the contribution!