feat(telemetry): use zero-width space to avoid clutter (#1754)

# What does this PR do?
Before 
<img width="858" alt="image"
src="https://github.com/user-attachments/assets/6cefb1ae-5603-4818-85ea-a0c337b986bc"
/>

Note the redundant 'llama-stack' in front of every span

## Test Plan
<img width="1171" alt="image"
src="https://github.com/user-attachments/assets/bdc5fd5b-ff1f-4f10-8b40-cff2ea93dd1f"
/>

llama_stack/templates/cerebras/run.yaml

@@ -79,7 +79,7 @@ providers:
   - provider_id: meta-reference
     provider_type: inline::meta-reference
     config:
-      service_name: ${env.OTEL_SERVICE_NAME:llama-stack}
+      service_name: "${env.OTEL_SERVICE_NAME:\u200B}"
       sinks: ${env.TELEMETRY_SINKS:console,sqlite}
       sqlite_db_path: ${env.SQLITE_DB_PATH:~/.llama/distributions/cerebras/trace_store.db}
   tool_runtime: