feat: record token usage for inference API (#1300)

# What does this PR do?
Inference router computes the token usage related metrics for all
providers and returns the metrics as part of response and also logs to
telemetry.

## Test Plan
LLAMA_STACK_DISABLE_VERSION_CHECK=true llama stack run
~/.llama/distributions/fireworks/fireworks-run.yaml

```
curl --request POST \
  --url http://localhost:8321/v1/inference/chat-completion \
  --header 'content-type: application/json' \
  --data '{
  "model_id": "meta-llama/Llama-3.1-70B-Instruct",
  "messages": [
    {
      "role": "user",
      "content": {
        "type": "text",
        "text": "where do humans live"
      }
    }
  ],
  "stream": false
}' | jq .
{
  "metrics": [
    {
      "trace_id": "yjv1tf0jS1evOyPm",
      "span_id": "WqYKvg0_",
      "timestamp": "2025-02-27T18:55:10.770903Z",
      "attributes": {
        "model_id": "meta-llama/Llama-3.1-70B-Instruct",
        "provider_id": "fireworks"
      },
      "type": "metric",
      "metric": "prompt_tokens",
      "value": 10,
      "unit": "tokens"
    },
    {
      "trace_id": "yjv1tf0jS1evOyPm",
      "span_id": "WqYKvg0_",
      "timestamp": "2025-02-27T18:55:10.770916Z",
      "attributes": {
        "model_id": "meta-llama/Llama-3.1-70B-Instruct",
        "provider_id": "fireworks"
      },
      "type": "metric",
      "metric": "completion_tokens",
      "value": 411,
      "unit": "tokens"
    },
    {
      "trace_id": "yjv1tf0jS1evOyPm",
      "span_id": "WqYKvg0_",
      "timestamp": "2025-02-27T18:55:10.770919Z",
      "attributes": {
        "model_id": "meta-llama/Llama-3.1-70B-Instruct",
        "provider_id": "fireworks"
      },
      "type": "metric",
      "metric": "total_tokens",
      "value": 421,
      "unit": "tokens"
    }
  ],
  "completion_message": {
    "role": "assistant",
    "content": "Humans live in various parts of the world, inhabiting almost every continent, country, and region. Here's a breakdown of where humans live:\n\n1. **Continents:** Humans inhabit all seven continents:\n\t* Africa\n\t* Antarctica (research stations only)\n\t* Asia\n\t* Australia\n\t* Europe\n\t* North America\n\t* South America\n2. **Countries:** There are 196 countries recognized by the United Nations, and humans live in almost all of them.\n3. **Regions:** Humans live in diverse regions, including:\n\t* Deserts (e.g., Sahara, Mojave)\n\t* Forests (e.g., Amazon, Congo)\n\t* Grasslands (e.g., Prairies, Steppes)\n\t* Mountains (e.g., Himalayas, Andes)\n\t* Oceans (e.g., coastal areas, islands)\n\t* Tundras (e.g., Arctic, sub-Arctic)\n4. **Cities and towns:** Many humans live in urban areas, such as cities and towns, which are often located near:\n\t* Coastlines\n\t* Rivers\n\t* Lakes\n\t* Mountains\n5. **Rural areas:** Some humans live in rural areas, such as:\n\t* Villages\n\t* Farms\n\t* Countryside\n6. **Islands:** Humans inhabit many islands, including:\n\t* Tropical islands (e.g., Hawaii, Maldives)\n\t* Arctic islands (e.g., Greenland, Iceland)\n\t* Continental islands (e.g., Great Britain, Ireland)\n7. **Extreme environments:** Humans also live in extreme environments, such as:\n\t* High-altitude areas (e.g., Tibet, Andes)\n\t* Low-altitude areas (e.g., Death Valley, Dead Sea)\n\t* Areas with extreme temperatures (e.g., Arctic, Sahara)\n\nOverall, humans have adapted to live in a wide range of environments and ecosystems around the world.",
    "stop_reason": "end_of_turn",
    "tool_calls": []
  },
  "logprobs": null
}
```

```
 LLAMA_STACK_CONFIG=fireworks pytest -s -v tests/integration/inference

======================================================================== short test summary info =========================================================================
FAILED tests/integration/inference/test_text_inference.py::test_text_chat_completion_tool_calling_tools_not_in_request[txt=8B:vis=11B-inference:chat_completion:tool_calling_tools_absent-True] - ValueError: Unsupported tool prompt format: ToolPromptFormat.json
FAILED tests/integration/inference/test_text_inference.py::test_text_chat_completion_tool_calling_tools_not_in_request[txt=8B:vis=11B-inference:chat_completion:tool_calling_tools_absent-False] - ValueError: Unsupported tool prompt format: ToolPromptFormat.json
FAILED tests/integration/inference/test_vision_inference.py::test_image_chat_completion_non_streaming[txt=8B:vis=11B] - fireworks.client.error.InvalidRequestError: {'error': {'object': 'error', 'type': 'invalid_request_error', 'message': 'Failed to decode image cannot identify image f...
FAILED tests/integration/inference/test_vision_inference.py::test_image_chat_completion_streaming[txt=8B:vis=11B] - fireworks.client.error.InvalidRequestError: {'error': {'object': 'error', 'type': 'invalid_request_error', 'message': 'Failed to decode image cannot identify image f...
========================================================= 4 failed, 16 passed, 23 xfailed, 17 warnings in 44.36s =========================================================
```

llama_stack/providers/inline/telemetry/meta_reference/telemetry.py

@@ -73,6 +73,7 @@ class TelemetryAdapter(TelemetryDatasetMixin, Telemetry):
     def __init__(self, config: TelemetryConfig, deps: Dict[str, Any]) -> None:
         self.config = config
         self.datasetio_api = deps.get(Api.datasetio)
+        self.meter = None
 
         resource = Resource.create(
             {
@@ -171,6 +172,8 @@ class TelemetryAdapter(TelemetryDatasetMixin, Telemetry):
         return _GLOBAL_STORAGE["gauges"][name]
 
     def _log_metric(self, event: MetricEvent) -> None:
+        if self.meter is None:
+            return
         if isinstance(event.value, int):
             counter = self._get_or_create_counter(event.metric, event.unit)
             counter.add(event.value, attributes=event.attributes)