This PR adds the coverage support to the platform with the LLVM/clang only, without GCC.

Background
This change is essential for FreeBSD as the Fluent Bit development platform. The master branch as of f8bd0ce does not build on FreeBSD 14.0-RELEASE because of the strerror_r(3) signature dependency on glibc. I am addressing this issue in parallel, in which the code coverage is required to prove the fix.

New CMake Build Option(s)

• FLB_COVERAGE_TYPE
  • Configure the coverage type.
    • gcov: (default) gcov; native for GCC, compatible with LLVM/clang.
    • llvm: LLVM; native for LLVM/clang.
  • FLB_COVERAGE is required.

Change(s) to CMAKE_C_FLAGS

• Add -fprofile-instr-generate -fcoverage-mapping.
  These are the standard CFLAGS options to enable the code coverage profile instrumentation on LLVM/clang.

Change(s) to Test Execution Environment

• Add LLVM_PROFILE_FILE=${CMAKE_CURRENT_BINARY_DIR}/(testname).profraw.
  • Output the raw profile data to the file distinct for each test.
  • Limitation: A runtime_shell test can execute at most one test binary.
    • Fulfilled as of f8bd0ce.

Out of Scope

• Collect the raw profile data files into a single profile data file by llvm-profdata.
• Generate the reports by llvm-cov.

Tested Environment

• Debian x86_64 12.2
  • Debian clang version 14.0.6

Testing
Before we can approve your change; please submit the following in a comment:

• N/A Example configuration file for the change
  This change does not add any new features to the Fluent Bit binary.
• Debug log output from testing the change
  • logs-fluent-bit-topic-llvm-coverage.zip: The archive of the log files and sample LLVM coverage report.
    • logs-fluent-bit-topic-llvm-coverage/
      • fluent-bit-cmake-topic-llvm-coverage-debian_x86_64_12_2.log: The cmake log.
        • Command line: env CC=clang-14 CXX=clang++-14 CPP=clang-cpp-14 LD=ld.lld RANLIB=llvm-ranlib-14 cmake -DFLB_COVERAGE=true -DFLB_COVERAGE_TYPE=llvm -DFLB_DEV=true -DFLB_VALGRIND=true -DFLB_TESTS_RUNTIME=true ..
      • fluent-bit-make-topic-llvm-coverage-debian_x86_64_12_2.log: The make log.
        • Command line: env CC=clang-14 CXX=clang++-14 CPP=clang-cpp-14 LD=ld.lld RANLIB=llvm-ranlib-14 make
      • fluent-bit-ctest-topic-llvm-coverage-debian_x86_64_12_2.log: The ctest log.
        • Command line: ctest
        • Remarks:
          • 1 failure.
            • flb-rt-out_td: The test environment problem, I cannot make the valid test configuration.
          • 2 cases stopped manually by SIGTERM.
            • in_syslog_uds_dgram_plaintext_expect.sh, in_syslog_uds_stream_plaintext_expect.sh: These are known to stall for more than 5 minutes, found during the test rehearsal. The results after SIGTERM are both successful.
      • fluent-bit-find-profraw-topic-llvm-coverage-debian_x86_64_12_2.log: The LLVM profile files found after ctest.
        • Command line: find tests -type f -name '*.profraw'
      • fluent-bit-llvm-profdata-cov-topic-llvm-coverage-debian_x86_64_12_2.log: The LLVM profile data collection and coverage report generation.
        • Command line: Please refer to the log file.
      • llvm-cov-test-topic-llvm-coverage-debian_x86_64_12_2.tar.xz: The LLVM coverage report in HTML.
• N/A Attached Valgrind output that shows no leaks or memory corruption was found
  This change does not touch any C sources.

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

This change does not touch the packaging.

Documentation

• Documentation required for this feature
  Doc PR: installation: sources: Add FLB_COVERAGE and FLB_COVERAGE_TYPE. fluent-bit-docs#1289

Backporting

• Backport to latest stable release.
  There is a fix depending on this PR. (pal: platform abstraction layer and strerror_r(3) wrapper #8390)

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.