refactor: clean up Monoscope Gorilla Mux middleware

[iamonah]

Refactor Gorilla Mux middleware for cleaner and more accurate telemetry capture. This replaces the httptest.NewRecorder-based approach with a custom responseRecorder that handles implicit 200 OK responses, optimizes body capture, simplifies request handling, and ensures compatibility with Monoscope and OpenTelemetry.

Closes #

How to test

1. Set up a local Gorilla Mux server using the updated monoscopegorilla.Middleware.
2. Test key scenarios:
   • Handler with explicit WriteHeader(201).
   • Handler writing body without WriteHeader (should imply 200 OK).
   • Handler with no body (e.g., HEAD or empty response).
3. Verify:
   • Telemetry spans are generated correctly.
   • Status codes are accurate (including fallbacks).
   • Bodies are captured based on config flags (CaptureRequestBody, CaptureResponseBody).
   • No issues with duplicate writes or headers.

Checklist

• Make sure you have described your changes and added all relevant screenshots or data.
• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests).
• Make sure to add/update documentation regarding your changes (or request one from the team).
• You are NOT deprecating/removing a feature.