use logging instead of prints (#499)

# What does this PR do?

This PR moves all print statements to use logging. Things changed:
- Had to add `await start_trace("sse_generator")` to server.py to
actually get tracing working. else was not seeing any logs
- If no telemetry provider is provided in the run.yaml, we will write to
stdout
- by default, the logs are going to be in JSON, but we expose an option
to configure to output in a human readable way.

llama_stack/providers/inline/agents/meta_reference/persistence.py

@@ -5,7 +5,7 @@
 # the root directory of this source tree.
 
 import json
-
+import logging
 import uuid
 from datetime import datetime
 
@@ -15,6 +15,8 @@ from pydantic import BaseModel
 
 from llama_stack.providers.utils.kvstore import KVStore
 
+log = logging.getLogger(__name__)
+
 
 class AgentSessionInfo(BaseModel):
     session_id: str
@@ -78,7 +80,7 @@ class AgentPersistence:
                 turn = Turn(**json.loads(value))
                 turns.append(turn)
             except Exception as e:
-                print(f"Error parsing turn: {e}")
+                log.error(f"Error parsing turn: {e}")
                 continue
         turns.sort(key=lambda x: (x.completed_at or datetime.min))
         return turns