Enable or disabled auto instrumentation (#137)

* Enable or disabled auto instrumentation
This is done through the OpenTelemetry\SDK\Sdk::isInstrumentationDisabled method

* Add NAME public constant to instrumentation classes

* Implement class_exist

* Introduce warning

* Remove unrelated changes

* Remove balnk line

* Suppress psalm warnings

* Add configuration documentation

---------

Co-authored-by: Przemyslaw Delewski <[email protected]>

Author: xvilo

README.md

@@ -49,3 +49,14 @@ From Symfony subdirectory:
 $ composer install
 $ ./vendor/bin/phpunit tests
 ```
+
+## Configuration
+
+Parts of this auto-instrumentation library can be configured, more options are available throught the
+[General SDK Configuration](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/sdk-environment-variables.md#general-sdk-configuration):
+
+| Name                                | Default value | Values                  | Example | Description                                                                     |
+|-------------------------------------|---------------|-------------------------|---------|---------------------------------------------------------------------------------|
+| OTEL_PHP_DISABLED_INSTRUMENTATIONS  | []            | Instrumentation name(s) | symfony | Disable one or more installed auto-instrumentations, names are comma seperated. |
+
+Configurations can be provided as environment variables, or via `php.ini` (or a file included by `php.ini`)

_register.php

@@ -4,6 +4,11 @@
 
 use OpenTelemetry\Contrib\Instrumentation\Symfony\HttpClientInstrumentation;
 use OpenTelemetry\Contrib\Instrumentation\Symfony\SymfonyInstrumentation;
+use OpenTelemetry\SDK\Sdk;
+
+if (class_exists(Sdk::class) && Sdk::isInstrumentationDisabled(SymfonyInstrumentation::NAME) === true) {
+    return;
+}
 
 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);

src/SymfonyInstrumentation.php

@@ -19,6 +19,8 @@
 
 final class SymfonyInstrumentation
 {
+    public const NAME = 'symfony';
+
     public static function register(): void
     {
         $instrumentation = new CachedInstrumentation('io.opentelemetry.contrib.php.symfony');