Be less restrictive when autoloading instrumentation libraries (#133)

* Be less restrictive when autoloading instrumentation libraries
Emit a warning through trigger_error instead of using an assert. This will allow one
to use the libraries in environments where the extension might not be available.
However in case of issues, it will be clear through logs why code might not be
instrumented

* Implement early return style

Author: xvilo

_register.php

@@ -2,6 +2,12 @@
 
 declare(strict_types=1);
 
-assert(extension_loaded('otel_instrumentation'));
+use OpenTelemetry\Contrib\Instrumentation\Symfony\SymfonyInstrumentation;
 
-\OpenTelemetry\Contrib\Instrumentation\Symfony\SymfonyInstrumentation::register();
+if (extension_loaded('otel_instrumentation') === false) {
+    trigger_error('The otel_instrumentation extension must be loaded in order to autoload the OpenTelemetry Laravel auto-instrumentation', E_USER_WARNING);
+
+    return;
+}
+
+SymfonyInstrumentation::register();