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/inference/meta_reference/inference.py

@@ -5,6 +5,7 @@
 # the root directory of this source tree.
 
 import asyncio
+import logging
 
 from typing import AsyncGenerator, List
 
@@ -25,6 +26,7 @@ from .config import MetaReferenceInferenceConfig
 from .generation import Llama
 from .model_parallel import LlamaModelParallelGenerator
 
+log = logging.getLogger(__name__)
 # there's a single model parallel process running serving the model. for now,
 # we don't support multiple concurrent requests to this process.
 SEMAPHORE = asyncio.Semaphore(1)
@@ -49,7 +51,7 @@ class MetaReferenceInferenceImpl(Inference, ModelRegistryHelper, ModelsProtocolP
         # verify that the checkpoint actually is for this model lol
 
     async def initialize(self) -> None:
-        print(f"Loading model `{self.model.descriptor()}`")
+        log.info(f"Loading model `{self.model.descriptor()}`")
         if self.config.create_distributed_process_group:
             self.generator = LlamaModelParallelGenerator(self.config)
             self.generator.start()