optimize resource creation (#1210)

* reduce resource creation from sdk autoloader
- instead of creating new resources from various factories, create them once from sdk autoloader and
pass them to the factories (most of which already supported this).
- check env before php.ini for config, since this is the more popular approach and saves some cycles
- cache header from OtlpUtil, to save a couple of calls to Sdk detector
* adding benchmark for resource creation

Author: brettmc

OtlpUtil.php

@@ -35,11 +35,16 @@ public static function method(string $signal): string
      */
     public static function getUserAgentHeader(): array
     {
-        $resource = (new Sdk())->getResource();
+        static $header;
+        if ($header === null) {
+            $resource = (new Sdk())->getResource();
+
+            $header = ['User-Agent' => sprintf(
+                'OTel OTLP Exporter PHP/%s',
+                $resource->getAttributes()->get(ResourceAttributes::TELEMETRY_SDK_VERSION) ?: 'unknown'
+            )];
+        }
 
-        return ['User-Agent' => sprintf(
-            'OTel OTLP Exporter PHP/%s',
-            $resource->getAttributes()->get(ResourceAttributes::TELEMETRY_SDK_VERSION) ?: 'unknown'
-        )];
+        return $header;
     }
 }