Include environment/service/version as common metric tags.

service/src/main/java/org/whispersystems/textsecuregcm/configuration/DatadogConfiguration.java

@@ -20,6 +20,10 @@ public class DatadogConfiguration {
   @NotNull
   private Duration step = Duration.ofSeconds(10);
 
+  @JsonProperty
+  @NotBlank
+  private String environment;
+
   public String getApiKey() {
     return apiKey;
   }
@@ -27,4 +31,8 @@ public class DatadogConfiguration {
   public Duration getStep() {
     return step;
   }
+
+  public String getEnvironment() {
+    return environment;
+  }
 }