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

[Ben Kochie]

For `modtile_cached_tiles_total`, I was kinda guessing based on the variable names. I'm also fine with `modtile_tiles_total`, that makes sense to me.

`duration` in the name is there to differentiate it from `timestamp`. But it's fine to drop `duration` if you want, it's somewhat implied.

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