Tidy up OTEL plugin typing

Explicitly annotate, and return a copy of the configuration instead of a
mutable reference.

Author: ndevenish

src/workflows/util/zocalo/configuration.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from typing import ClassVar, TypedDict
+
 from marshmallow import fields
 from zocalo.configuration import PluginSchema
 
@@ -11,21 +13,25 @@
 class OTEL:
     """A Zocalo configuration plugin to pre-populate OTELTracing config defaults"""
 
+    class _OTELConfig(TypedDict, total=False):
+        endpoint: str
+        timeout: int
+
+    # Store configuration for access by services
+    config: ClassVar[_OTELConfig] = {}
+
     class Schema(PluginSchema):
         host = fields.Str(required=True)
         port = fields.Int(required=True)
         timeout = fields.Int(required=False, load_default=10)
 
-    # Store configuration for access by services
-    config = {}
-
     @staticmethod
     def activate(configuration):
         # Build the full endpoint URL
         endpoint = f"https://{configuration['host']}:{configuration['port']}/v1/traces"
         OTEL.config["endpoint"] = endpoint
         OTEL.config["timeout"] = configuration.get("timeout", 10)
-        return OTEL.config
+        return dict(OTEL.config)
 
 
 class Stomp: