Add observability to consume mode

[simonswine]

The consume mode currently is not exposing any status of how successful its remote_write requests.

Ideally it should expose metrics about the outcome of the remote_write requests in a similar way as the "produce" mode does.

The pulsar client itself registers some metrics about consume the messages from the bus.

Their metrics definition is here:

prometheus-pulsar-remote-write/pkg/app/metrics.go

Line 8 in ed7be59

type metrics struct {

Ideally the metric vector includes the tenant ID if set, so we can figure which tenant fails to deliver remote_write requests

[simonswine]

Currently the consume mode itself is not merged yet #34, but should be fairly soon

/cc @56quarters