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

[ǝɹʇʇɐʃǝ◖ xıʃǝɟ]

@tomhughes , does this look good for you as well? After a thumbs-up from you, I'd merge. Thanks!

```
# HELP modtile_http_responses_total Number of HTTP responses by response code
# TYPE modtile_http_responses_total counter
modtile_http_responses_total{status="200"} 314
modtile_http_responses_total{status="304"} 99
modtile_http_responses_total{status="404"} 0
modtile_http_responses_total{status="503"} 0
modtile_http_responses_total{status="5XX"} 0
modtile_http_responses_total{status="other"} 0
# HELP modtile_tiles_total Tiles served
# TYPE modtile_tiles_total counter
modtile_tiles_total{age="fresh",rendered="no"} 332
modtile_tiles_total{age="old",rendered="no"} 0
modtile_tiles_total{age="outdated",rendered="no"} 0
modtile_tiles_total{age="fresh",rendered="yes"} 81
modtile_tiles_total{age="old",rendered="attempted"} 0
modtile_tiles_total{age="outdated",rendered="attempted"} 0
# HELP modtile_zoom_responses_total Tiles served by zoom level
# TYPE modtile_zoom_responses_total counter
modtile_zoom_responses_total{zoom="00"} 0
modtile_zoom_responses_total{zoom="01"} 0
modtile_zoom_responses_total{zoom="02"} 0
modtile_zoom_responses_total{zoom="03"} 0
modtile_zoom_responses_total{zoom="04"} 177
modtile_zoom_responses_total{zoom="05"} 0
modtile_zoom_responses_total{zoom="06"} 65
modtile_zoom_responses_total{zoom="07"} 0
modtile_zoom_responses_total{zoom="08"} 105
modtile_zoom_responses_total{zoom="09"} 0
modtile_zoom_responses_total{zoom="10"} 66
modtile_zoom_responses_total{zoom="11"} 0
modtile_zoom_responses_total{zoom="12"} 0
modtile_zoom_responses_total{zoom="13"} 0
modtile_zoom_responses_total{zoom="14"} 0
modtile_zoom_responses_total{zoom="15"} 0
modtile_zoom_responses_total{zoom="16"} 0
modtile_zoom_responses_total{zoom="17"} 0
modtile_zoom_responses_total{zoom="18"} 0
modtile_zoom_responses_total{zoom="19"} 0
modtile_zoom_responses_total{zoom="20"} 0
# HELP modtile_tile_reads_total Tiles served from the tile buffer
# TYPE modtile_tile_reads_total counter
modtile_tile_reads_total{zoom="00"} 0
modtile_tile_reads_total{zoom="01"} 0
modtile_tile_reads_total{zoom="02"} 0
modtile_tile_reads_total{zoom="03"} 0
modtile_tile_reads_total{zoom="04"} 177
modtile_tile_reads_total{zoom="05"} 0
modtile_tile_reads_total{zoom="06"} 65
modtile_tile_reads_total{zoom="07"} 0
modtile_tile_reads_total{zoom="08"} 105
modtile_tile_reads_total{zoom="09"} 0
modtile_tile_reads_total{zoom="10"} 66
modtile_tile_reads_total{zoom="11"} 0
modtile_tile_reads_total{zoom="12"} 0
modtile_tile_reads_total{zoom="13"} 0
modtile_tile_reads_total{zoom="14"} 0
modtile_tile_reads_total{zoom="15"} 0
modtile_tile_reads_total{zoom="16"} 0
modtile_tile_reads_total{zoom="17"} 0
modtile_tile_reads_total{zoom="18"} 0
modtile_tile_reads_total{zoom="19"} 0
modtile_tile_reads_total{zoom="20"} 0
# HELP modtile_tile_reads_seconds_total Tile buffer duration
# TYPE modtile_tile_reads_seconds_total counter
modtile_tile_reads_seconds_total{zoom="00"} 0.000000
modtile_tile_reads_seconds_total{zoom="01"} 0.000000
modtile_tile_reads_seconds_total{zoom="02"} 0.000000
modtile_tile_reads_seconds_total{zoom="03"} 0.000000
modtile_tile_reads_seconds_total{zoom="04"} 0.006143
modtile_tile_reads_seconds_total{zoom="05"} 0.000000
modtile_tile_reads_seconds_total{zoom="06"} 0.002044
modtile_tile_reads_seconds_total{zoom="07"} 0.000000
modtile_tile_reads_seconds_total{zoom="08"} 0.002948
modtile_tile_reads_seconds_total{zoom="09"} 0.000000
modtile_tile_reads_seconds_total{zoom="10"} 0.002253
modtile_tile_reads_seconds_total{zoom="11"} 0.000000
modtile_tile_reads_seconds_total{zoom="12"} 0.000000
modtile_tile_reads_seconds_total{zoom="13"} 0.000000
modtile_tile_reads_seconds_total{zoom="14"} 0.000000
modtile_tile_reads_seconds_total{zoom="15"} 0.000000
modtile_tile_reads_seconds_total{zoom="16"} 0.000000
modtile_tile_reads_seconds_total{zoom="17"} 0.000000
modtile_tile_reads_seconds_total{zoom="18"} 0.000000
modtile_tile_reads_seconds_total{zoom="19"} 0.000000
modtile_tile_reads_seconds_total{zoom="20"} 0.000000
# HELP modtile_layer_responses_total Layer respones
# TYPE modtile_layer_responses_total counter
modtile_layer_responses_total{layer="/renderd-example/tiles/",status="200"} 413
modtile_layer_responses_total{layer="/renderd-example/tiles/",status="404"} 0
```

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