[Tile-serving] [openstreetmap/mod_tile] Add a Prometheus metrics endpoint (#222)

[ǝɹʇʇɐʃǝ◖ xıʃǝɟ]

Tested it and seems to work fine:

* `promtool` is happy
* I can see changes in the metrics when putting some traffik on the `mod_tile`

This looks good to me and seems to be ready to be merged.

Many thanks, @SuperQ.

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