appsec/grpc: fix rpc message blocking

[Julio-Guerra]

What does this PR do?

• Fix gRPC message blocking
• Increase the test coverage of gRPC blocking so that this path is now properly tested. Only IP-, user- and metadata-blocking was tested.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

JIRA: APPSEC-53472

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-06-04 12:29:34

Comparing candidate commit fc5c6eb in PR branch julio.guerra/fix-grpc-blocking with baseline commit 6126fb6 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 43 metrics, 1 unstable metrics.