[l2geth]: How to expose prometheus metrics?

[qezz]

Describe the issue

I run an RPC node on mainnet, and I want to monitor it with prometheus. But it's not really clear how to do this.

I can see from the code, and from the --help output that --metrics flag exists, but there are no --metrics.port or --metrics.addr.

There's also METRICS_ENABLE=true env var, but I'm not sure if it does anything. And I also couldn't find any configuration for the listen address and port.

To Reproduce
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Expected behavior

CLI options and env vars exist to listen on the certain addr:port

Screenshots
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Additional context
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