feat: optimize observability logic with internal enablement flags

[appleboy]

• Add internal optimization flags to detect if metrics, tracer, or logger are enabled
• Update code paths to use these flags, conditionally invoking metrics, tracing, and logging logic only when enabled
• Add a new test file to verify observability flag behavior and configuration
• Ensure tracing, metrics, and logging components are not called when explicitly disabled

[Copilot]

Pull request overview

This pull request introduces performance optimizations for observability components by adding internal boolean flags that detect whether metrics, tracer, or logger components are enabled. The optimization conditionally invokes observability methods only when the respective component is not a no-op implementation, avoiding unnecessary interface method call overhead.

Changes:

• Added three internal boolean flags (metricsEnabled, tracerEnabled, loggerEnabled) to the Client struct
• Implemented type assertion-based detection during client initialization to set these flags
• Updated all observability call sites to check flags before invoking methods
• Added comprehensive tests to verify flag detection and conditional behavior

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
retry.go	Added internal optimization flags to Client struct, implemented flag detection in NewClient using type assertions against nop implementations, and wrapped all observability method calls with conditional checks based on these flags
observability_flags_test.go	New test file that verifies flag detection for different configurations (default, all disabled, individual components) and includes a test to ensure tracer is not called when disabled

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