feat(ingester): Remote Write format

[JPBotelho]

The current ingestion format is comically naive, we're just fetching /metrics, adding a timestamp and appending to a file. We should: try to convert these /metrics scrapes into HTTP Write packets.

Then, it comes naturally that the ingester should support being an HTTP Remote Write target (eg an agent fetches metrics and remote_writes them to the ingester, which saves it all in a single file)