Prometheus and opentelemetry for metrics and tracing

[JoTurk]

Core SFU library for counters/gauges/histograms and tracing for ops visibility. I think we should support two setups:

1. just Prometheus metrics.
2. OTeL metrics / tracing + proms exporter.

tasks:

• Define a basic security module for exposed /metrics. disabled by default, localhost by default if enabled, and the ability to add auth.
• Add prometheus registry and /metrics via promhttp.
• Instrument base Go metrics and process metrics
• Add labels policy to avoid high cardinality (e.g., room names not as labels)
• structure for setting up tracers using OTeL, but modular for any other backend in the future.