Add telemetry & Prometheus metrics endpoint, tests, docs, and lint fixes

[cybcon]

Summary

This PR adds a telemetry package to collect and expose Prometheus-compatible metrics (CPU, memory, load average, read/write counters), integrates metrics wiring into the server, and includes unit tests, examples, and documentation. It also contains linting updates and small persistence fixes.

What I changed

• Added telemetry components:
  • src/app/lib/telemetry/metrics_datastore.py
  • src/app/lib/telemetry/metrics_server.py
  • src/app/lib/telemetry/prometheus_metrics.py
  • src/app/lib/telemetry/__init__.py
• Integrated metrics into the server:
  • src/app/modbus_server.py (metrics wiring / endpoint integration)
  • src/app/modbus_server.json (config updates)
• Tests:
  • Added/updated telemetry tests in tests/ (e.g., tests/test_metrics_datastore.py, tests/test_metrics_server.py, tests/test_prometheus_metrics.py)
• Examples & docs:
  • Added examples/metrics_example.txt
  • Updated examples/abb_coretec_example.json, examples/test.json, examples/udp.json
  • Updated README.md with metrics documentation
• Persistence & misc:
  • Tweaks to src/app/lib/register_persistence/__init__.py
• Build / CI / linting:
  • .pre-commit-config.yaml updated; linter fixes applied
  • src/requirements.txt updated (ensure psutil present)
• Containers:
  • Dockerfile and Dockerfile.test updated to expose metrics endpoint port

Files changed (high-level)

• src/app/lib/telemetry/* (new/modified)
• src/app/modbus_server.py
• src/app/modbus_server.json
• src/app/lib/register_persistence/__init__.py
• src/requirements.txt
• examples/metrics_example.txt, examples/*.json
• Dockerfile, Dockerfile.test
• .pre-commit-config.yaml
• tests/* (new/updated telemetry tests)
• README.md

Testing

• Run the full test suite:

pytest

• Verify metrics endpoint (default path and port described in updated README.md) returns Prometheus-format metrics.