Add prometheus metrics

[msbarry]

Add a new command-line --admin-port option that lets you optionally exposes prometheus /metrics endpoint on an admin port. By default prometheus exposes some basic info about the go process, but this PR also adds a singe custom pmtiles_cache_size metric as an example that returns the current number of directories in the cache.

When I build on my m1 mac this increases the build size 974kb from 48 to 49mb.

Here's what the `/metrics` endpoint looks like by default

```diff # HELP go_gc_duration_seconds A summary of the pause duration of garbage collection cycles. # TYPE go_gc_duration_seconds summary go_gc_duration_seconds{quantile="0"} 2.9333e-05 go_gc_duration_seconds{quantile="0.25"} 0.000164209 go_gc_duration_seconds{quantile="0.5"} 0.000168833 go_gc_duration_seconds{quantile="0.75"} 0.000411001 go_gc_duration_seconds{quantile="1"} 0.000411001 go_gc_duration_seconds_sum 0.000773376 go_gc_duration_seconds_count 4 # HELP go_goroutines Number of goroutines that currently exist. # TYPE go_goroutines gauge go_goroutines 14 # HELP go_info Information about the Go environment. # TYPE go_info gauge go_info{version="go1.21.1"} 1 # HELP go_memstats_alloc_bytes Number of bytes allocated and still in use. # TYPE go_memstats_alloc_bytes gauge go_memstats_alloc_bytes 9.335696e+06 # HELP go_memstats_alloc_bytes_total Total number of bytes allocated, even if freed. # TYPE go_memstats_alloc_bytes_total counter go_memstats_alloc_bytes_total 1.3326648e+07 # HELP go_memstats_buck_hash_sys_bytes Number of bytes used by the profiling bucket hash table. # TYPE go_memstats_buck_hash_sys_bytes gauge go_memstats_buck_hash_sys_bytes 5753 # HELP go_memstats_frees_total Total number of frees. # TYPE go_memstats_frees_total counter go_memstats_frees_total 39740 # HELP go_memstats_gc_sys_bytes Number of bytes used for garbage collection system metadata. # TYPE go_memstats_gc_sys_bytes gauge go_memstats_gc_sys_bytes 4.2428e+06 # HELP go_memstats_heap_alloc_bytes Number of heap bytes allocated and still in use. # TYPE go_memstats_heap_alloc_bytes gauge go_memstats_heap_alloc_bytes 9.335696e+06 # HELP go_memstats_heap_idle_bytes Number of heap bytes waiting to be used. # TYPE go_memstats_heap_idle_bytes gauge go_memstats_heap_idle_bytes 3.19488e+06 # HELP go_memstats_heap_inuse_bytes Number of heap bytes that are in use. # TYPE go_memstats_heap_inuse_bytes gauge go_memstats_heap_inuse_bytes 1.2730368e+07 # HELP go_memstats_heap_objects Number of allocated objects. # TYPE go_memstats_heap_objects gauge go_memstats_heap_objects 36634 # HELP go_memstats_heap_released_bytes Number of heap bytes released to OS. # TYPE go_memstats_heap_released_bytes gauge go_memstats_heap_released_bytes 3.072e+06 # HELP go_memstats_heap_sys_bytes Number of heap bytes obtained from system. # TYPE go_memstats_heap_sys_bytes gauge go_memstats_heap_sys_bytes 1.5925248e+07 # HELP go_memstats_last_gc_time_seconds Number of seconds since 1970 of last garbage collection. # TYPE go_memstats_last_gc_time_seconds gauge go_memstats_last_gc_time_seconds 1.706444075934168e+09 # HELP go_memstats_lookups_total Total number of pointer lookups. # TYPE go_memstats_lookups_total counter go_memstats_lookups_total 0 # HELP go_memstats_mallocs_total Total number of mallocs. # TYPE go_memstats_mallocs_total counter go_memstats_mallocs_total 76374 # HELP go_memstats_mcache_inuse_bytes Number of bytes in use by mcache structures. # TYPE go_memstats_mcache_inuse_bytes gauge go_memstats_mcache_inuse_bytes 12000 # HELP go_memstats_mcache_sys_bytes Number of bytes used for mcache structures obtained from system. # TYPE go_memstats_mcache_sys_bytes gauge go_memstats_mcache_sys_bytes 15600 # HELP go_memstats_mspan_inuse_bytes Number of bytes in use by mspan structures. # TYPE go_memstats_mspan_inuse_bytes gauge go_memstats_mspan_inuse_bytes 208320 # HELP go_memstats_mspan_sys_bytes Number of bytes used for mspan structures obtained from system. # TYPE go_memstats_mspan_sys_bytes gauge go_memstats_mspan_sys_bytes 211848 # HELP go_memstats_next_gc_bytes Number of heap bytes when next garbage collection will take place. # TYPE go_memstats_next_gc_bytes gauge go_memstats_next_gc_bytes 1.3295448e+07 # HELP go_memstats_other_sys_bytes Number of bytes used for other system allocations. # TYPE go_memstats_other_sys_bytes gauge go_memstats_other_sys_bytes 1.589167e+06 # HELP go_memstats_stack_inuse_bytes Number of bytes in use by the stack allocator. # TYPE go_memstats_stack_inuse_bytes gauge go_memstats_stack_inuse_bytes 851968 # HELP go_memstats_stack_sys_bytes Number of bytes obtained from system for stack allocator. # TYPE go_memstats_stack_sys_bytes gauge go_memstats_stack_sys_bytes 851968 # HELP go_memstats_sys_bytes Number of bytes obtained from system. # TYPE go_memstats_sys_bytes gauge go_memstats_sys_bytes 2.2842384e+07 # HELP go_threads Number of OS threads created. # TYPE go_threads gauge go_threads 14 + # HELP pmtiles_cache_size Current number or directories in the cache + # TYPE pmtiles_cache_size gauge + pmtiles_cache_size 3 # HELP promhttp_metric_handler_requests_in_flight Current number of scrapes being served. # TYPE promhttp_metric_handler_requests_in_flight gauge promhttp_metric_handler_requests_in_flight 1 # HELP promhttp_metric_handler_requests_total Total number of scrapes by HTTP status code. # TYPE promhttp_metric_handler_requests_total counter promhttp_metric_handler_requests_total{code="200"} 2 promhttp_metric_handler_requests_total{code="500"} 0 promhttp_metric_handler_requests_total{code="503"} 0 ```

[bdon]

Did this update go.mod and go.sum as well?

[msbarry]

Did this update go.mod and go.sum as well?

If I just do go mod tidy it moves github.com/prometheus/client_golang v1.15.1 from a transitive to direct dependency, but if I do go get github.com/prometheus/client_golang it updates it to 1.18. Not exactly sure which other dependency uses it too. Any preference?