check for mongo extension (#194)

if platform requirements are ignored, then trying to load this module causes a fatal error.
that's particularly bad because it happens during composer autoloading, so add an extra
check and do not try to run without the extension.

src/Instrumentation/MongoDB/_register.php

@@ -8,5 +8,8 @@
 if (class_exists(Sdk::class) && Sdk::isInstrumentationDisabled(MongoDBInstrumentation::NAME) === true) {
     return;
 }
+if (!extension_loaded('mongodb')) {
+    return;
+}
 
 MongoDBInstrumentation::register();