Prevent propagator packages from throwing fatal error if sdk not inst…

…alled (#1198)

* Prevent propagator extensions from failing if sdk not installed

* Remove KnownValues dependency

Also prevents failing for cloudtrace: sdk < 1.0.1 and jaeger: sdk < 1.0.3.

_register.php

@@ -3,14 +3,17 @@
 declare(strict_types=1);
 
 use OpenTelemetry\Extension\Propagator\B3\B3Propagator;
-use OpenTelemetry\SDK\Common\Configuration\KnownValues;
 use OpenTelemetry\SDK\Registry;
 
+if (!class_exists(Registry::class)) {
+    return;
+}
+
 Registry::registerTextMapPropagator(
-    KnownValues::VALUE_B3,
+    'b3',
     B3Propagator::getB3SingleHeaderInstance()
 );
 Registry::registerTextMapPropagator(
-    KnownValues::VALUE_B3_MULTI,
+    'b3multi',
     B3Propagator::getB3MultiHeaderInstance()
 );