fix: tracing fixes for trace context propogation across coroutines (#1522)

# What does this PR do?
This PR has two fixes needed for correct trace context propagation
across asycnio boundary
Fix 1: Start using context vars to store the global trace context.
This is needed since we cannot use the same trace context across
coroutines since the state is shared. each coroutine
should have its own trace context so that each of it can start storing
its state correctly.
Fix 2: Start a new span for each new coroutines started for running
shields to keep the span tree clean


## Test Plan

### Integration tests with server
LLAMA_STACK_DISABLE_VERSION_CHECK=true llama stack run
~/.llama/distributions/together/together-run.yaml
LLAMA_STACK_CONFIG=http://localhost:8321 pytest -s --safety-shield
meta-llama/Llama-Guard-3-8B --text-model
meta-llama/Llama-3.1-8B-Instruct
server logs:
https://gist.github.com/dineshyv/51ac5d9864ed031d0d89ce77352821fe
test logs:
https://gist.github.com/dineshyv/e66acc1c4648a42f1854600609c467f3
 
### Integration tests with library client
LLAMA_STACK_CONFIG=fireworks pytest -s --safety-shield
meta-llama/Llama-Guard-3-8B --text-model
meta-llama/Llama-3.1-8B-Instruct

logs: https://gist.github.com/dineshyv/ca160696a0b167223378673fb1dcefb8

### Apps test with server:
```
LLAMA_STACK_DISABLE_VERSION_CHECK=true llama stack run ~/.llama/distributions/together/together-run.yaml
python -m examples.agents.e2e_loop_with_client_tools localhost 8321
```
server logs:
https://gist.github.com/dineshyv/1717a572d8f7c14279c36123b79c5797
app logs:
https://gist.github.com/dineshyv/44167e9f57806a0ba3b710c32aec02f8

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

@@ -10,6 +10,7 @@ from typing import List
 
 from llama_stack.apis.inference import Message
 from llama_stack.apis.safety import Safety, SafetyViolation, ViolationLevel
+from llama_stack.providers.utils.telemetry import tracing
 
 log = logging.getLogger(__name__)
 
@@ -32,15 +33,14 @@ class ShieldRunnerMixin:
         self.output_shields = output_shields
 
     async def run_multiple_shields(self, messages: List[Message], identifiers: List[str]) -> None:
-        responses = await asyncio.gather(
-            *[
-                self.safety_api.run_shield(
+        async def run_shield_with_span(identifier: str):
+            async with tracing.span(f"run_shield_{identifier}"):
+                return await self.safety_api.run_shield(
                     shield_id=identifier,
                     messages=messages,
                 )
-                for identifier in identifiers
-            ]
-        )
+
+        responses = await asyncio.gather(*[run_shield_with_span(identifier) for identifier in identifiers])
         for identifier, response in zip(identifiers, responses, strict=False):
             if not response.violation:
                 continue