perf: batch log/metric delivery instead of one blocking HTTP call per event

[mleczakm]

Why

Every logger()->info() / metrics()->set() / metrics()->mutate() call fired its own fully synchronous Guzzle request with a 5s timeout, directly on the caller's code path - the send()/sendMetric() methods carried a literal // TODO: queue, retry, batching. Under high log/metric volume, or when the logdash API is slow or unreachable, this could stall the host application for up to 5 seconds per call, serially.

What

• RequestQueue (new): buffers items in memory and flushes them when a batch-size threshold is hit, when a flush interval has elapsed, or when the PHP process/request shuts down (via register_shutdown_function). Includes bounded retry for transient failures.
• HttpLogSync: now buffers log lines and posts them to POST /logs/batch (the same endpoint the Node SDK already uses) instead of issuing one POST /logs per line.
• Metrics: now dispatches queued metric updates concurrently via a GuzzleHttp\Pool (curl_multi under the hood) instead of one sequential blocking PUT per update. Failures stay isolated per item and are not retried at the batch level, since mutate is not idempotent and retrying a whole batch could double-apply an increment.
• Split connect_timeout (2s) from the overall timeout (5s) on both clients, so a fully unreachable host fails fast on connect instead of burning the whole 5s budget just to establish a connection.
• httpClient is now an injectable constructor param on both classes (optional, defaults unchanged) so they're unit-testable with MockHandler instead of hitting the network.

Public API (LogSync/BaseMetrics interfaces, send()/set()/mutate() signatures) is unchanged - this is transport-layer only. Both classes gained an optional flush() method for callers (e.g. long-running workers) that want to force a send before going idle, instead of waiting on the batch size/interval/shutdown triggers.

Proof

A MockHandler-based benchmark: 200 send() calls now produce 8 HTTP requests instead of 200, with all 200 log lines delivered intact (sequence numbers preserved). New tests cover queue batching/retry logic, the /logs/batch payload shape, and that one failing metric in a batch doesn't block the others.

Testing

• composer test - 14/14 passing (was 5/5; added RequestQueueTest, HttpLogSyncTest, MetricsTest)
• composer phpstan - level 8, no errors
• composer cs - PSR-12, clean
• ./vendor/bin/parallel-lint src tests - no syntax errors