Add CORS to jaeger query endpoint and make spans nested

Adds an nginx in front of the query endpoint so we can use stalk
without faffing with browser extension to bypass CORS.

Also make the spans correctly have the call membership span as parent,
which they didn't because we hadn't set the span at the point we made
the context.

config/otel_dev/docker-compose.yaml

@@ -21,3 +21,9 @@ services:
       - "55670:55679" # zpages extension
     depends_on:
       - jaeger-all-in-one
+  nginx:
+    image: nginxinc/nginx-unprivileged:latest
+    volumes:
+      - ./nginx_otel.conf:/etc/nginx/conf.d/default.conf:ro
+    ports:
+      - "16687:8080"