Add a minimal msgpack serialiser for ccf

[cjen1-msft]

For #7858 we want to export more logs more efficiently.
I proposed that we should support msgpack export, and this is a minimal msgpack serialiser.

Here is a benchmark of the serialiser against nlohmann (json dump) and rapidjson (as an 'optimal baseline')

 Name (* = baseline)      |   Dim   |  Total ms |  ns/op  |Baseline| Ops/second
--------------------------|--------:|----------:|--------:|-------:|----------:
 json_dump *              |     100 |     1.631 |   16306 |      - |    61323.7
 rapidjson_write          |     100 |     0.142 |    1423 |  0.087 |   702622.2
 msgpack_encode           |     100 |     0.189 |    1894 |  0.116 |   527779.7
 json_dump *              |    1000 |    10.267 |   10266 |      - |    97401.4
 rapidjson_write          |    1000 |     1.886 |    1886 |  0.184 |   530199.1
 msgpack_encode           |    1000 |     2.280 |    2280 |  0.222 |   438575.1
 json_dump *              |   10000 |    97.549 |    9754 |      - |   102512.6
 rapidjson_write          |   10000 |    12.481 |    1248 |  0.128 |   801223.8
 msgpack_encode           |   10000 |    16.415 |    1641 |  0.168 |   609205.9

[Copilot]

Pull request overview

This PR introduces a minimal, header-only MessagePack encoder under src/msgpack/ (including fluentd in_forward EventTime ext type support) to enable more efficient log export as part of the broader observability/log-export work (#7858).

Changes:

• Added a C++20 header-only msgpack encoder (write_* primitives, container headers, and FluentdEventTime).
• Added unit tests (boundary tables + property tests) and differential tests using nlohmann::json::from_msgpack as an oracle.
• Added a libFuzzer harness and integrated the new unit/fuzz tests into the build.

Custom instructions used:

• .github/copilot-instructions.md
• .github/instructions/reviewing.instructions.md

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
src/msgpack/encode.h	New header-only msgpack encoder API, error model, and Fluentd EventTime support
src/msgpack/endian.h	Big-endian writing helper for msgpack wire format
src/msgpack/test/encode_test.cpp	Property/boundary tests for smallest-format-wins encoding and EventTime layout
src/msgpack/test/differential_test.cpp	Differential encode-vs-nlohmann decode tests + fluentd message-mode pinned vector
src/msgpack/test/gen.h	Shared generator + script driver used by tests and fuzz harness
src/msgpack/test/fuzz_script_test.cpp	Deterministic/canned script tests pinning specific composite byte layouts
src/msgpack/test/format_introspect.h	Test helper to classify msgpack first-byte format families
src/msgpack/test/msgpack_fuzz.cpp	libFuzzer harness for encoder round-trip validation via nlohmann oracle
CMakeLists.txt	Registers msgpack_test unit test and msgpack_fuzz_test fuzz target

[Amaury Chamayou (achamayou)]

are we able to switch to C++23 on Azure Linux 3 already?

[Eddy Ashton (eddyashton)]

I'd prefer to have the FluentdEventTime type defined here, next to the only function that actually needs it? And probably-maybe in a separate header, so it's clearly an opt-in for the places that want it rather than a core part of the generic msgpack API.