Listen on localhost for Gitaly's Prometheus

What does this MR do?

This MR updates Gitaly's Prometheus endpoint to listen on localhost instead of all interfaces, which on macOS causes a popup asking to allow this behaviour:

As well as stopping the popup from occurring, it's generally best practice for services to listen on localhost unless explicitly required otherwise.

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

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