Add Datadog integration (#6289)

* Event integration updates and cleanups

* Add Datadog integration

* Update README to include link to Datadog PR

* Move doc update into the Datadog PR; Fix empty message on ArgumentException

* Adjust exception message

Co-authored-by: Matt Bishop <mbishop@bitwarden.com>

* Removed unnecessary nullable enable; Moved Docs link to PR into this PR

* Remove unnecessary nullable enable calls

---------

Co-authored-by: Matt Bishop <mbishop@bitwarden.com>

dev/servicebusemulator_config.json

@@ -34,6 +34,9 @@
               },
               {
                 "Name": "events-hec-subscription"
+              },
+              {
+                "Name": "events-datadog-subscription"
               }
             ]
           },
@@ -81,6 +84,20 @@
                     }
                   }
                 ]
+              },
+              {
+                "Name": "integration-datadog-subscription",
+                "Rules": [
+                  {
+                    "Name": "datadog-integration-filter",
+                    "Properties": {
+                      "FilterType": "Correlation",
+                      "CorrelationFilter": {
+                        "Label": "datadog"
+                      }
+                    }
+                  }
+                ]
               }
             ]
           }