feat: Add OpenTelemetry environment variable and options configuration helpers.

[chalmerlowe]

This PR introduces a foundational configuration layer for integrating OpenTelemetry (OTel) observability into Google Cloud Python client libraries. It establishes the google.api_core.observability module, which is responsible for resolving telemetry configuration settings in a standardized, hierarchical manner (and ultimately enabling traces, metrics, and logs).

Intent & Motivation

To provide a consistent observability experience across all Google Cloud Python clients, we are standardizing how telemetry signals (traces, metrics, logs) are enabled or disabled. This module parses and resolves configuration from multiple sources, falling back gracefully, so that users can control telemetry globally or on a per-service basis.

Changes Included

• Configuration Resolution Logic: Added the is_signal_enabled function in the new observability package to resolve telemetry enablement settings based on a precedence hierarchy:

Resolves settings in the following order of precedence:
1. Programmatic overrides in client_options (checks tracer_provider)
2. Language-wide Environment Variable: GOOGLE_CLOUD_PYTHON_TRACING_ENABLED
(natively checks for an EXPERIMENTAL prefix variant first)
3. Default fallback

• Experimental Fallbacks: Implemented fallback logic that automatically checks for EXPERIMENTAL prefixes within the environment variable definitions to support early-stage adoption and rollout phases safely.

• Test Coverage: Provided test coverage using pytest for precedence rules and environmental scenarios.

[gemini-code-assist]

Code Review

This pull request introduces observability environment variable and client options resolution helpers under a new observability module, including the is_signal_enabled function and corresponding unit tests. Feedback is provided to improve the resolution of client options by avoiding direct __dict__ access (which fails for classes using __slots__ or properties) in favor of getattr, and replacing rstrip('s') with a safer suffix removal method to prevent unintended character stripping.