Make datadog optional on staging

Author: bdach

README.md

@@ -39,7 +39,8 @@ Depending on environment, this service changes behaviour with respect to interac
    - Local (filesystem-based) beatmap storage is used.
    - No purge requests to beatmap mirrors are made on beatmap updates.
    - Developer exception pages & API docs are disabled.
-   - Sentry & Datadog integrations are mandatory.
+   - Sentry integration is mandatory.
+   - Datadog integration is optional.
 - `ASPNETCORE_ENVIRONMENT=Production`:
    - S3-based beatmap storage is used.
    - Purge requests to beatmap mirrors are made on beatmap updates.
@@ -62,5 +63,5 @@ Depending on environment, this service changes behaviour with respect to interac
 | `S3_CENTRAL_BUCKET_NAME`     | The name of the S3 bucket to use for storing beatmap packages and versioned files.                                                                 |       ⚠ In production environment       | None          |
 | `S3_BEATMAPS_BUCKET_NAME`    | The name of the S3 bucket to use for storing .osu beatmap files.                                                                                   |       ⚠ In production environment       | None          |
 | `SENTRY_DSN`                 | A valid Sentry DSN to use for logging application events.                                                                                          |  ⚠ In staging & production environment  | None          | 
-| `DD_AGENT_HOST`              | A hostname pointing to a Datadog agent instance to which metrics should be reported.                                                               |  ⚠ In staging & production environment  | None          | 
+| `DD_AGENT_HOST`              | A hostname pointing to a Datadog agent instance to which metrics should be reported.                                                               |       ⚠ In production environment       | None          | 
 

osu.Server.BeatmapSubmission/Program.cs

@@ -73,12 +73,6 @@ public static void Main(string[] args)
                                                             + "Please set the value of this variable to a valid Sentry DSN to use for logging events.");
                     }
 
-                    if (AppSettings.DatadogAgentHost == null)
-                    {
-                        throw new InvalidOperationException("DD_AGENT_HOST environment variable not set. "
-                                                            + "Please set the value of this variable to a valid hostname of a Datadog agent.");
-                    }
-
                     break;
                 }