Support referencing env vars in the postgresql configuration

Adds support for referencing `$DATABASE_HOSTNAME` and similar env vars as the value of the configuration; the `$` will be the trigger in this case and checks the environment if the key exists before using it, otherwise falling back on the value given. This then allows for hosts/users/pw and so on that would start with `$`

Author: helgi

newrelic_python_agent/plugins/postgresql.py

@@ -277,10 +277,16 @@ def connection_arguments(self):
         filtered_args = ["name", "superuser", "relation_stats"]
         args = {}
         for key in set(self.config) - set(filtered_args):
+            value = self.config[key]
+            # If value starts with $ and exists as an env var, use that value
+            if value[0] == "$":
+                # strip the $, check if the env var exists, otherwise fallback
+                value = os.getenv(value, value)
+                
             if key == 'dbname':
-                args['database'] = self.config[key]
+                args['database'] = value
             else:
-                args[key] = self.config[key]
+                args[key] = value
         return args
 
     def poll(self):