statsmanager: use rust-based prometheus implementation

[jkarneges]

This replaces the C++ Prometheus server code in StatsManager with calls to the Rust-based Prometheus implementation via FFI.

The changes are fairly straightforward:

• If Prometheus metrics are enabled, instantiate Rust-based Prometheus server and common metrics instances.
• Whenever combinedReport is updated, copy the values to the common metrics.

As a bonus, the event loop no longer needs to account for related I/O registrations (since the underlying PrometheusServer manages them in a dedicated thread) so that math is simplified.

Manually tested on Linux, to see the process metrics working alongside the common metrics:

# HELP connection_connected Number of concurrent connections
# TYPE connection_connected gauge
connection_connected 0
# HELP connection_minute Number of minutes clients have been connected
# TYPE connection_minute counter
connection_minute 0
# HELP message_received Number of messages received by the publish API
# TYPE message_received counter
message_received 1
# HELP message_sent Number of messages sent to clients
# TYPE message_sent counter
message_sent 0
# HELP process_cpu_seconds_total Total user and system CPU time spent in seconds.
# TYPE process_cpu_seconds_total counter
process_cpu_seconds_total 0
# HELP process_max_fds Maximum number of open file descriptors.
# TYPE process_max_fds gauge
process_max_fds 1024
# HELP process_open_fds Number of open file descriptors.
# TYPE process_open_fds gauge
process_open_fds 55
# HELP process_resident_memory_bytes Resident memory size in bytes.
# TYPE process_resident_memory_bytes gauge
process_resident_memory_bytes 20639744
# 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 1789083449
# HELP process_threads Number of OS threads in the process.
# TYPE process_threads gauge
process_threads 4
# HELP process_virtual_memory_bytes Virtual memory size in bytes.
# TYPE process_virtual_memory_bytes gauge
process_virtual_memory_bytes 232411136
# HELP request_received Number of requests received
# TYPE request_received counter
request_received 0