[ecbadeaux]

Summary

Replaces the single shared buffer in the writer with per-thread buffering and one background sender thread. Recording metrics from several threads is now safe and fast in buffered mode. This also adds a configurable flush interval and full-buffer policy.

Changes

• Per-thread buffered writer (libs/writer/writer_wrapper/buffered_writer.{h,cpp})
  • Each recording thread appends lines to its own buffer and hands it to a single sender thread when the buffer is full. Threads don't touch each other's buffers.
  • Datagrams contain only whole lines and are never larger than the buffer size. The one exception is a single line that is longer than the buffer, which goes out on its own.
  • Lines from one thread keep their order.
  • Each thread can have up to 8 full buffers waiting for the sender.
  • When a thread exits, its lines go to the sender. When the process shuts down, everything still buffered is sent.
• New WriterConfig options (passed through Config → Registry → Writer::Initialize)
  • flushInterval: defaults to 3 s. The sender sends partly filled buffers at this interval so lines from quiet threads aren't held back. It must be between 0 (exclusive) and 30 s, or the constructor throws std::runtime_error.
  • fullBufferPolicy: what a thread does when it runs out of buffers.
    • Block (default): wait for the sender.
    • Drop: drop the line and log the drop count.
    • Send: the thread sends its oldest buffer itself.
• Writer refactor: removes the old mutex/condition-variable buffering. Writer::Write is now static, and the meter headers call Writer::Write(line) directly.
• UDSWriter: adds an optional receiverFullWait. On macOS a send can fail with "no buffer space"; the writer now retries with backoff for up to 250 ms in buffered mode instead of failing right away.
• Performance test: adds the full-buffer policies and flush intervals to the scenarios, report tables and CSV output.
• Docs: updates the top-level README.md and libs/writer/README.md to describe buffering behavior and settings.

Testing

• New buffered_writer_test suite (test_buffered_writer.cpp)
• Extended tests for WriterConfig, Config, Writer, UDSWriter and Registry
• Updated the writer performance test to cover the new settings