Flow: allow receiving remote_write metrics over HTTP

[rfratto]

The static mode supports sending metrics over HTTP to a metrics instance, but there is no Flow equivalent for this yet.

I see two paths:

1. Add an API endpoint for prometheus.remote_write that allows external processes to send metrics to it over the network (i.e., /components/prometheus.remote_write.default/remote_write)
2. Create a new component dedicated to receiving metrics over the network, which would allow it to be chained into other components for processing.

Option 2 is probably immediately preferable for its increased flexibility.

[thampiotr]

Proposing to take path 2 and the component to be called prometheus.source.api.