Tag manual OTel span with request user-agent for rid correlation

get_spans_list filters spans by matching the request rid embedded in
http.useragent; the manually-created OTel span didn't carry that tag,
so it was invisible to test assertions once OTel bridging is enabled.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: MilanGarnier

utils/build/docker/php/common/otel_drop_in_extract_and_make_distant_call.php

@@ -25,6 +25,7 @@
     ->setParent($context)
     ->setSpanKind(\OpenTelemetry\API\Trace\SpanKind::KIND_SERVER)
     ->startSpan();
+$span->setAttribute('http.useragent', $_SERVER['HTTP_USER_AGENT'] ?? '');
 $scope = $span->activate();
 
 // Make the distant call; auto-instrumentation propagates the active span context.