Full rewrite of collective.prometheus

[Sakoes]

A full rewrite of the @@metrics endpoint, modernizing it for current Prometheus practices.

What changed:

• Uses prometheus_client for exposition — proper Content-Type, Cache-Control, and OpenMetrics negotiation. Also mounts the standard PROCESS_COLLECTOR / PLATFORM_COLLECTOR.
• ZODB load/store metrics are now real counters (zodb_object_loads_total, zodb_object_stores_total), backed by a monotonic accumulator instead of a 15-second sampling window. Resets only on Zope restart — use rate() in dashboards.
• Cleaner labels: per-database series use a db label (not _<dbname> name suffixes); per-connection series use a connection label (fixes a bug where connection 0 collided with later samples under the old thread label).
• Renames: zope_cache_memory → zope_cache_objects (it always counted objects, not bytes).
• New gauges: zope_busy_threads, zope_request_queue_length.
• Concurrency fix: collector is built per-request instead of stashing request/context on a process-wide singleton, which raced under concurrent scrapes. Activity totals live in a module-level dict behind a lock.
• Robustness: each sub-collector is isolated — one failure no longer blanks the whole scrape.
• Cleanup: drops pre-2.5 threadframe fallback; tightens setup.py classifiers (Plone-only, not raw Zope2).

Breaking for anyone with existing dashboards — metric names, labels, and types have changed. Tagged as 2.0.0 (UNRELEASED).

Example output

curl http://localhost:8082/@@metrics
# HELP process_virtual_memory_bytes Virtual memory size in bytes.
# TYPE process_virtual_memory_bytes gauge
process_virtual_memory_bytes 5.33516288e+08
# HELP process_resident_memory_bytes Resident memory size in bytes.
# TYPE process_resident_memory_bytes gauge
process_resident_memory_bytes 2.18329088e+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 1.78032678674e+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 12.71
# HELP process_open_fds Number of open file descriptors.
# TYPE process_open_fds gauge
process_open_fds 24.0
# HELP process_max_fds Maximum number of open file descriptors.
# TYPE process_max_fds gauge
process_max_fds 1024.0
# HELP python_info Python platform information
# TYPE python_info gauge
python_info{implementation="CPython",major="3",minor="11",patchlevel="13",version="3.11.13"} 1.0
# HELP zope_total_objects Number of objects in the ZODB.
# TYPE zope_total_objects gauge
zope_total_objects{db="main"} 23393.0
# HELP zope_cache_objects Objects currently held in the ZODB cache.
# TYPE zope_cache_objects gauge
zope_cache_objects{db="main"} 11768.0
# HELP zope_cache_size Configured maximum objects per ZODB cache.
# TYPE zope_cache_size gauge
zope_cache_size{db="main"} 15000.0
# HELP zodb_connections Open ZODB connections.
# TYPE zodb_connections gauge
zodb_connections{db="main"} 0.0
# HELP zodb_object_loads_total Total ZODB object loads.
# TYPE zodb_object_loads_total counter
zodb_object_loads_total{db="main"} 3816.0
# HELP zodb_object_stores_total Total ZODB object stores.
# TYPE zodb_object_stores_total counter
zodb_object_stores_total{db="main"} 7937.0
# HELP zope_connection_active_objects Active (non-ghost) objects in a ZODB connection cache.
# TYPE zope_connection_active_objects gauge
zope_connection_active_objects{connection="0",db="main"} 196.0
zope_connection_active_objects{connection="1",db="main"} 178.0
zope_connection_active_objects{connection="2",db="main"} 182.0
zope_connection_active_objects{connection="3",db="main"} 11187.0
zope_connection_active_objects{connection="4",db="main"} 25.0
# HELP zope_connection_total_objects Total objects in a ZODB connection cache.
# TYPE zope_connection_total_objects gauge
zope_connection_total_objects{connection="0",db="main"} 1296.0
zope_connection_total_objects{connection="1",db="main"} 1238.0
zope_connection_total_objects{connection="2",db="main"} 1238.0
zope_connection_total_objects{connection="3",db="main"} 12496.0
zope_connection_total_objects{connection="4",db="main"} 531.0