feat: add prometheus metrics to /metrics

[tgalske]

Summary

This PR adds the Prometheus Python client as a dependency, and serves up the out-of-the-box metrics on the /metrics endpoint.

The metrics mounting code was taken almost directly from Prometheus' documentation on how to add metrics to a FastAPI application: https://prometheus.github.io/client_python/exporting/http/fastapi-gunicorn/

Since this would be my first contribution and in an effort to keep the change on the small side, this PR does not expose any custom metrics.

Testing

• make docker-test, make check pass
• Manually verified the metrics by running make docker-start-api and observing the metrics at http://localhost:8000/metrics

Example output

# HELP python_gc_objects_collected_total Objects collected during gc
# TYPE python_gc_objects_collected_total counter
python_gc_objects_collected_total{generation="0"} 10468.0
python_gc_objects_collected_total{generation="1"} 4124.0
python_gc_objects_collected_total{generation="2"} 2134.0
# HELP python_gc_objects_uncollectable_total Uncollectable objects found during GC
# TYPE python_gc_objects_uncollectable_total counter
python_gc_objects_uncollectable_total{generation="0"} 0.0
python_gc_objects_uncollectable_total{generation="1"} 0.0
python_gc_objects_uncollectable_total{generation="2"} 0.0
# HELP python_gc_collections_total Number of times this generation was collected
# TYPE python_gc_collections_total counter
python_gc_collections_total{generation="0"} 952.0
python_gc_collections_total{generation="1"} 86.0
python_gc_collections_total{generation="2"} 6.0
# HELP python_info Python platform information
# TYPE python_info gauge
python_info{implementation="CPython",major="3",minor="11",patchlevel="10",version="3.11.10"} 1.0
# HELP process_virtual_memory_bytes Virtual memory size in bytes.
# TYPE process_virtual_memory_bytes gauge
process_virtual_memory_bytes 4.717744128e+09
# HELP process_resident_memory_bytes Resident memory size in bytes.
# TYPE process_resident_memory_bytes gauge
process_resident_memory_bytes 8.1219584e+08
# HELP process_start_time_seconds Start time of the process since unix epoch in seconds.
# TYPE process_start_time_seconds gauge
process_start_time_seconds 1.72882768797e+09
# HELP process_cpu_seconds_total Total user and system CPU time spent in seconds.
# TYPE process_cpu_seconds_total counter
process_cpu_seconds_total 9.059999999999999
# HELP process_open_fds Number of open file descriptors.
# TYPE process_open_fds gauge
process_open_fds 20.0
# HELP process_max_fds Maximum number of open file descriptors.
# TYPE process_max_fds gauge
process_max_fds 1.048576e+06

[tgalske]

Pointing out that /metrics calls are not being logged already:

unstructured-api/prepline_general/api/app.py

Lines 121 to 124 in b43fef0

	# Filter out /metrics noise
	class MetricsCheckFilter(logging.Filter):
	def filter(self, record: logging.LogRecord) -> bool:
	return record.getMessage().find("/metrics") == -1