impl(o11y): introduce body.size

[diegomarquezp]

Description

This PR enhances the OpenTelemetry integration for HTTP JSON transports to explicitly unpack and record the http.response.body.size attributes globally inside google-cloud-java.

Key Enhancements:

• Safeguarded Extractions: Solidified extractContentLength in both SpanTracer and TestApiTracer to parse List-type strings safely and silently skip payload sizes when chunked serialization omits Content-Length headers entirely!
• True-Positive Validation Mappings: In ITOtelTracing, cleanly simulates exception routing natively while introducing algorithmic boundary estimations isAtLeast(expectedMagnitude * 0.85) using actual Message object size fingerprints.

Testing Instructions

1. Run SpanTracerTest.java
2. Run ITOtelTracing in showcase to test true-positive generation bounding.

[gemini-code-assist]

Summary of Changes

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This pull request significantly improves the OpenTelemetry integration for HTTP JSON transports by introducing the http.response.body.size attribute. This enhancement provides more granular observability into the size of HTTP response bodies, which is crucial for performance monitoring and debugging. The changes involve updating core tracing components to capture and propagate this new attribute, along with robust parsing logic for content length headers and comprehensive test coverage to ensure accuracy.

Highlights

• OpenTelemetry Integration: Introduced the http.response.body.size attribute for OpenTelemetry spans in HTTP JSON transports to provide granular observability into response body sizes.
• Content-Length Extraction Robustness: Enhanced the extractContentLength logic in SpanTracer and TestApiTracer to safely parse Content-Length from various header formats, including List-type strings, and to handle cases where chunked encoding omits the header.
• ApiTracer Propagation: Integrated ApiTracer into HttpJsonCallOptions and updated HttpJsonCallContext to correctly propagate the tracer throughout the call context.
• Response Header Callback: Added a responseHeadersReceived callback to the ApiTracer interface and its implementations (SpanTracer, TestApiTracer) to capture response header information, enabling the recording of body size.
• Comprehensive Testing: Implemented new test cases in HttpJsonClientCallImplTest to validate the accurate recording of response body sizes for both unary and server-streaming calls, and updated ITOtelTracing to include assertions for the new attribute.
• Mock Service Enhancement: Modified MockHttpService to explicitly add Content-Length headers to mock HTTP responses for non-streaming calls, ensuring accurate testing of the new observability feature.

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[gemini-code-assist]

Code Review

This pull request implements tracing for HTTP JSON calls, specifically to record the response body size. This involves adding ApiTracer support to HttpJsonCallOptions and HttpJsonCallContext, updating HttpJsonClientCallImpl to report response headers to the tracer, and enhancing ApiTracer and SpanTracer to process Content-Length from response headers. New tests were added to verify this functionality. Feedback includes addressing code duplication in the extractContentLength method, correcting a flawed test case in SpanTracerTest for bad Content-Length formats, and improving the robustness of a test in ITOtelTracing by asserting the presence of the http.response.body.size attribute.