logging: add a marshalled-bytes counter

[moonchen]

Adds proxy.process.log.marshalled_bytes, a counter that helps with estimating the cost of logging on an ATS instance.

[Copilot]

Pull request overview

Adds a new process metric to quantify how many bytes ATS marshals for logging, helping operators estimate logging overhead.

Changes:

• Register a new counter metric: proxy.process.log.marshalled_bytes.
• Track marshalled byte volume during LogObject::log() for log-access entries.
• Extend the logging stats block (LogsStatsBlock) with the new counter pointer.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/proxy/logging/LogObject.cc	Increments the new marshalled_bytes counter during log entry preparation.
src/proxy/logging/LogConfig.cc	Registers the new proxy.process.log.marshalled_bytes counter with the Metrics system.
include/proxy/logging/LogConfig.h	Adds marshalled_bytes to LogsStatsBlock so it can be referenced from logging code.

Comments suppressed due to low confidence (1)

src/proxy/logging/LogObject.cc:645

• proxy.process.log.marshalled_bytes is incremented before _checkout_write() succeeds. If the log entry is later dropped because no buffer is available (e.g., entry exceeds buffer payload and the function returns Log::FAIL), this counter still increases even though nothing was actually marshalled into a log buffer. Move the increment to after the buffer checkout succeeds (and keep it gated on lad) so the counter reflects bytes for successfully-buffered log-access entries.

    Metrics::Counter::increment(log_rsb.marshalled_bytes, bytes_needed);
  } else if (!text_entry.empty()) {
    bytes_needed = INK_ALIGN_DEFAULT(text_entry.size() + 1); // must include null terminator.
  }

[moonchen]

[approve ci autest]