[SMT-0001]: task-local metrics factory proposal implementation draft

[kukushechkin]

Add task-local factory override for metrics to enable testing code that creates and emits metrics without global state
pollution. The factory is captured at metric creation time and used for the metric's entire lifetime.

Motivation:

Testing code that creates and emits metrics currently requires either:

• Bootstrapping a test factory globally, which prevents parallel test execution and pollutes test state across tests.
• Passing factory explicitly through the API boundaries, which pollutes API of internal methods and libraries.

Modifications:

Add MetricsSystem.with(factory:) method that binds a factory to task-local context. Metrics created within this
context use the task-local factory instead of the global factory.

Result:

Metrics can be used and tests without global metrics factory bootstrapping and explicit factory propagation though dependency injection.