Add withLog() for logging alongside metrics

[jszobody]

Summary

• Adds Metric::withLog(string $message, string $level = 'info') to log a message when a metric is flushed
• The metric's extra includes 'message' => $logMessage
• The log context includes ['event' => $metricName] merged with the metric's extra
• Logging happens at flush time via flushMetricLogs() so all extra data is finalized
• Fixes PostHog driver to batch metrics like other drivers instead of sending immediately in add(), which broke fluent chaining (Metrics::create('event')->setExtra([...]))

Usage

Metrics::create("my_event")
    ->withLog("Something happened", "info")
    ->setExtra(['key' => 'value']);

Test plan

• withLog() stores message and level, defaults to info
• getExtra() includes message key when log is set
• Log fires at flush time with correct level, message, and context
• All 81 tests pass