[datadog-monitor] Allow default_tags values to contain colons (bugfix).

Only separate the key before the first colon, all subsequent colons are
considered to be part of the value as stated in datadog docs.

datadog/provider.go

@@ -470,12 +470,14 @@ func tagDiff(ctx context.Context, d *schema.ResourceDiff, meta interface{}) erro
 		kv := strings.Split(tag.(string), ":")
 		var key, value string
 		switch len(kv) {
+		case 0:
+			return fmt.Errorf("invalid tag: '%s'", tag)
 		case 2:
 			key, value = kv[0], kv[1]
 		case 1:
 			key, value = kv[0], ""
 		default:
-			return fmt.Errorf("invalid tag: '%s'", tag)
+            key, value = kv[0], strings.Join(kv[1:], ":")
 		}
 		tags[key] = value
 	}

datadog/tests/cassettes/TestAccDatadogMonitor_DefaultTags.freeze

@@ -1 +1 @@
-2024-07-12T15:58:49.631984+02:00
+2024-12-04T13:18:38.138439+01:00