Add system_id to Prometheus metrics

What does this MR do?

This MR adds a system_id label to all Prometheus metrics that currently have a runner label.

Why was this MR needed?

So that metrics can be filtered by system_id, like they can be for runner.

What's the best way to test this MR?

1. Add the following line to the top of your config.toml:

   listen_address = "127.0.0.1:9252"

2. Open http://127.0.0.1:9252/metrics

3. Notice that lines that have a runner label now also have a system_id label:

   

What are the relevant issue numbers?

#29542 (closed)