feat(otlp): callback-based HTTP exporter with in-flight limit

[LimiNode]

Summary

Replaces the blocking future.wait_for() HTTP export loop in OtlpHttpLogger with kurlyk callback API, enabling non-blocking batched exports up to a configurable max_in_flight_requests limit. Also introduces a new OtlpPayloadLogger backend that forms OTLP JSON payloads and passes them to a user-provided callback instead of sending HTTP itself.

Motivation

The previous implementation blocked the dedicated worker thread on every HTTP round-trip (future.wait_for(...) -> future.get()). This serialized all batch exports and prevented the worker from dequeuing new logs while a slow collector endpoint was being processed. Under backpressure the queue filled up and triggered drop_on_overflow even though the worker was idle.

Changes

OtlpHttpLogger.hpp

• Introduces OtlpHttpLoggerState (shared_ptr-held) with mutex, cv, http_in_flight and failed_exports counters.
• HTTP callback captures weak_ptr<OtlpHttpLoggerState> instead of this, preventing use-after-free if the logger is destroyed before the callback fires.
• worker_loop() no longer blocks inside export_batch(). It increments http_in_flight, calls submit_batch_async(), and immediately loops back to dequeue the next batch while http_in_flight < max_in_flight_requests.
• wait() now waits on http_in_flight == 0 && m_queue.empty() via the shared-state condition variable.
• stop() uses m_client.wait_requests() for graceful drain, or m_client.cancel_requests() when cancel_on_shutdown = true.
• Config moved to nested OtlpHttpLogger::Config with OtlpJsonFormatConfig format sub-member.

OtlpPayloadLogger.hpp (new)

• New callback-only backend that serializes batches to OTLP JSON and passes the payload string to a user-provided std::function<void(std::string)>.
• Supports sync and async modes, queue overflow drop policy, and FailedExportCount / DroppedLogCount diagnostics.
• Callback exceptions are caught (both sync and async) to prevent killing the worker thread.
• Config is nested OtlpPayloadLogger::Config sharing OtlpJsonFormatConfig format.

OtlpJsonFormatConfig.hpp (new)

• Extracted shared serialization settings (service/resource attributes, include_* flags, args_prefix) from the old standalone OtlpHttpLoggerConfig.
• Used by both OtlpHttpLogger, OtlpPayloadLogger, and build_otlp_logs_json_payload().

Deleted

• otlp/OtlpHttpLoggerConfig.hpp — replaced by OtlpHttpLogger::Config.
• otlp/OtlpPayloadLoggerConfig.hpp — replaced by OtlpPayloadLogger::Config.

CMakeLists.txt / tests/CMakeLists.txt

• Guards install(EXPORT) when OTLP is enabled (kurlyk target not exported).
• Raises C++ standard to C++17 when LOGIT_WITH_OTLP=ON (required by kurlyk).
• Adds target_include_directories for Simple-Web-Server headers used by OTLP HTTP integration tests.
• Adds otlp_payload_logger_test to the test list (guarded by LOGIT_WITH_OTLP).

New tests

otlp_http_logger_callback_test.cpp — 7 scenarios:

1. Single batch callback export — payload received and JSON structure validated.
2. max_in_flight=1 blocking — second batch is not submitted until the first callback completes.
3. max_in_flight=2 parallelism — two batches are submitted concurrently.
4. HTTP 500 failure counting — FailedExportCount increments correctly.
5. wait() blocks until callback completes — measured elapsed time >= server delay.
6. Graceful shutdown without UAF — slow server, cancel_on_shutdown=false, shutdown waits for callback and completes cleanly.
7. cancel_on_shutdown=true fast path — slow server, shutdown completes in < 1s.

otlp_payload_logger_test.cpp — 8 scenarios:

1. Sync mode callback receives valid JSON with "resourceLogs".
2. Sync throwing callback increments FailedExportCount without escaping from log().
3. Async batching — 5 messages coalesced into 1 payload.
4. Async queue overflow with drop_on_overflow=true — DroppedLogCount > 0.
5. wait() blocks until queue drain.
6. shutdown() stops worker cleanly without deadlock.
7. Slow async callback — wait() blocks until callback finishes (>= 400 ms).
8. Throwing async callback — worker survives, FailedExportCount increments.

Verification

• All 55 tests pass (0 failures).
• otlp_http_logger_callback_test PASS in ~3.9s.
• otlp_payload_logger_test PASS.
• No regressions in existing OTLP integration tests.

Scope risk

Moderate — the threading model changes from "worker thread blocks on HTTP" to "worker thread submits and moves on". The shared-state + weak_ptr pattern ensures safety, but concurrent callback delivery from kurlyk's NetworkWorker thread now decrements http_in_flight asynchronously.