feat: Add diagnostics library for tracking stats/counters

[allenporter]

Add a generic library that can be used to track counters, or elapsed time (e.g. how long it takes on average to connect to mqtt). This is a copy of https://github.com/allenporter/python-google-nest-sdm/blob/main/google_nest_sdm/diagnostics.py

This is an initial pass to add a few initial example metrics for MQTT, but we can add more as we need fine grained details in diagnostics. This will become more important with lazy device initialization as users may not see explicit error messages from the integration and we'll want them to provide diagnostics.

[Copilot]

Pull request overview

This PR adds a generic diagnostics library to track operational statistics and timing metrics across the codebase, with initial implementation focused on MQTT session metrics.

• Introduces a Diagnostics class supporting counter increments, timing measurements, and hierarchical subkey organization
• Integrates diagnostics tracking into MQTT sessions (connection attempts, subscriptions, message dispatches) and device manager operations
• Provides comprehensive test coverage for the diagnostics module and its integration points

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
roborock/diagnostics.py	New diagnostics module with counter, timer, and subkey support
roborock/mqtt/session.py	Adds diagnostics field to MqttParams dataclass
roborock/mqtt/roborock_session.py	Instruments MQTT operations with diagnostics tracking (connection, subscribe, publish, restart)
roborock/devices/device_manager.py	Integrates diagnostics into DeviceManager with device discovery tracking and data exposure
tests/test_diagnostics.py	Comprehensive unit tests for diagnostics functionality
tests/mqtt/test_roborock_session.py	Integration test validating MQTT diagnostics collection
tests/devices/test_device_manager.py	Integration test for device manager diagnostics

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