Combine GCP resource detectors (#181)

* Delete individual classes for each GCP resource type

* Combine all GCP resource detectors in a single class

* Add missing javadoc

* Fix Cloud Function detection

* Rename GCPComputeResource to GCPResource

* Fix GKE cluster location detection

GKE resource should either only have the zone or the region as their location.
Updated the source from where the location attribute is extracted.

* Avoid adding malformed location to attributes in GKE

Author: psx95

detectors/resources/src/main/java/com/google/cloud/opentelemetry/detectors/CloudRunResource.java

@@ -93,6 +93,11 @@ String getClusterName() {
     return getAttribute("instance/attributes/cluster-name");
   }
 
+  // Returns null on failure to retrieve from metadata server
+  String getClusterLocation() {
+    return getAttribute("instance/attributes/cluster-location");
+  }
+
   // Returns null on failure to retrieve from metadata server
   String getInstanceHostName() {
     return getAttribute("instance/hostname");