Prevent propagator packages from throwing fatal error if sdk not installed (#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.

Author: Nevay

_register.php

@@ -4,15 +4,18 @@
 
 use OpenTelemetry\Extension\Propagator\Jaeger\JaegerBaggagePropagator;
 use OpenTelemetry\Extension\Propagator\Jaeger\JaegerPropagator;
-use OpenTelemetry\SDK\Common\Configuration\KnownValues;
 use OpenTelemetry\SDK\Registry;
 
+if (!class_exists(Registry::class)) {
+    return;
+}
+
 Registry::registerTextMapPropagator(
-    KnownValues::VALUE_JAEGER,
+    'jaeger',
     JaegerPropagator::getInstance()
 );
 
 Registry::registerTextMapPropagator(
-    KnownValues::VALUE_JAEGER_BAGGAGE,
+    'jaeger-baggage',
     JaegerBaggagePropagator::getInstance()
 );