fix(config): use correct DD_* environment variables

The Datadog SDK requires DD_* environment variables (DD_API_KEY,
DD_APP_KEY, DD_SITE) to function correctly. The previous implementation
used DATADOG_* variable names which never worked.

This fix changes the configuration to use the correct DD_* variable
names that the Datadog SDK actually reads and uses.

Breaking Change: Users must update their environment variables from
DATADOG_* to DD_* naming convention. The old DATADOG_* variables were
never functional.

Required changes:
- DATADOG_API_KEY → DD_API_KEY
- DATADOG_APP_KEY → DD_APP_KEY
- DATADOG_SITE → DD_SITE (optional, defaults to datadoghq.com)

Author: jimmystridh

server.py

@@ -81,14 +81,17 @@ async def app_lifespan(server: FastMCP) -> AsyncIterator[AppContext]:
 
 
 def _load_config() -> DatadogConfig:
-    """Load Datadog configuration from environment"""
-    api_key = os.getenv("DATADOG_API_KEY")
-    app_key = os.getenv("DATADOG_APP_KEY")
-    site = os.getenv("DATADOG_SITE", "datadoghq.com")
-    
+    """Load Datadog configuration from environment
+
+    Note: Uses DD_* environment variables as required by the Datadog SDK.
+    """
+    api_key = os.getenv("DD_API_KEY")
+    app_key = os.getenv("DD_APP_KEY")
+    site = os.getenv("DD_SITE", "datadoghq.com")
+
     if not api_key or not app_key:
-        raise ValueError("DATADOG_API_KEY and DATADOG_APP_KEY must be set")
-        
+        raise ValueError("DD_API_KEY and DD_APP_KEY must be set")
+
     return DatadogConfig(api_key=api_key, app_key=app_key, site=site)