fead: add OpenTelemetry observability support for modular inputs

[wtobis-splunk]

Issue number
ADDON-85636

Summary
This PR introduces a new solnlib.observability module that adds an ObservabilityService for Splunk modular inputs and a LoggerMetricExporter for local/debug metric export. The service initializes standard event count and event byte counters, reads TA metadata from app.conf, attempts secure OTLP export to the local Spotlight collector with env/IPC-based port discovery, and degrades gracefully when observability is unavailable.

It also adds unit tests covering exporter behavior and OTLP setup/fallback paths, exports the new module through solnlib.__init__, and updates project dependencies and MkDocs references so the new observability API is documented and packaged correctly, and formally drops Python 3.7 support from the package configuration.

Testing

• Manual testing on MSCS
• Unit tests added

[kkedziak-splunk]

Suggested change

	# Copyright 2025 Splunk Inc.
	# Copyright 2026 Splunk Inc.

[wtobis-splunk]

Done, updated to 2026.

[kkedziak-splunk]

I am thinking about this import. It means that observability will be imported even if it is not used.

I checked the otel modules and importing them takes around 0,5-0,6 sec.

It's just a note. If that's ok, then we can leave it.

[wtobis-splunk]

Good point — removed observability from __init__.py to avoid the ~0.5s import overhead for users who don't need it. Users should import directly via from solnlib.observability import ....

[kkedziak-splunk]

Suggested change

	_Logger: TypeAlias = Union[logging.Logger, logging.LoggerAdapter]
	_Logger = Union[logging.Logger, logging.LoggerAdapter]

What about not adding a type to _Logger, in order to avoid typing_extensions? If we want to leave it, maybe we should add it as a dependency in pyproject.toml?

[wtobis-splunk]

Removed typing_extensions dependency and TypeAlias annotation — using plain _Logger = Union[...] instead.