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

[ǝɹʇʇɐʃǝ◖ xıʃǝɟ]

Tested and got error with: `curl http://localhost/metrics | promtool check metrics

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  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  4703  100  4703    0     0  4592k      0 --:--:-- --:--:-- --:--:-- 4592k
error while linting: text format parsing error in line 87: second TYPE line for metric name "modtile_layer_responses_total", or TYPE reported after samples


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