feat(tests): metrics tests (#3966)

# What does this PR do?
1. Make telemetry tests as easy as possible for users by expanding the
`SpanStub` data class and creating the `MetricStub` dataclass as a way
to consistently marshal telemetry data in test fixtures and unmarshal
and handle it in tests.
2. Structure server and client tests to always follow the same standards
for consistent testing experience by using the `SpanStub` and
`MetricStub` data class objects.
3. Enable Metrics Testing for completions endpoint
4. Correct token metrics to use histograms instead of counts to capture
tokens per request rather than a cumulative count of tokens over the
lifecycle of the server.

## Test Plan
These are tests

tests/integration/telemetry/collectors/in_memory.py

@@ -6,8 +6,6 @@
 
 """In-memory telemetry collector for library-client tests."""
 
-from typing import Any
-
 import opentelemetry.metrics as otel_metrics
 import opentelemetry.trace as otel_trace
 from opentelemetry import metrics, trace
@@ -19,46 +17,42 @@ from opentelemetry.sdk.trace.export.in_memory_span_exporter import InMemorySpanE
 
 import llama_stack.core.telemetry.telemetry as telemetry_module
 
-from .base import BaseTelemetryCollector, SpanStub
+from .base import BaseTelemetryCollector, MetricStub, SpanStub
 
 
 class InMemoryTelemetryCollector(BaseTelemetryCollector):
+    """In-memory telemetry collector for library-client tests.
+
+    Converts OpenTelemetry span objects to SpanStub objects to ensure
+    consistent interface with OTLP collector used in server mode.
+    """
+
     def __init__(self, span_exporter: InMemorySpanExporter, metric_reader: InMemoryMetricReader) -> None:
+        super().__init__()
         self._span_exporter = span_exporter
         self._metric_reader = metric_reader
 
-    def _snapshot_spans(self) -> tuple[Any, ...]:
+    def _snapshot_spans(self) -> tuple[SpanStub, ...]:
         spans = []
         for span in self._span_exporter.get_finished_spans():
-            trace_id = None
-            span_id = None
-            context = getattr(span, "context", None)
-            if context:
-                trace_id = f"{context.trace_id:032x}"
-                span_id = f"{context.span_id:016x}"
-            else:
-                trace_id = getattr(span, "trace_id", None)
-                span_id = getattr(span, "span_id", None)
-
-            stub = SpanStub(
-                span.name,
-                span.attributes,
-                getattr(span, "resource", None),
-                getattr(span, "events", None),
-                trace_id,
-                span_id,
-            )
-            spans.append(stub)
-
+            spans.append(self._create_span_stub_from_opentelemetry(span))
         return tuple(spans)
 
-    def _snapshot_metrics(self) -> Any | None:
+    def _snapshot_metrics(self) -> tuple[MetricStub, ...] | None:
         data = self._metric_reader.get_metrics_data()
-        if data and data.resource_metrics:
-            resource_metric = data.resource_metrics[0]
+        if not data or not data.resource_metrics:
+            return None
+
+        metric_stubs = []
+        for resource_metric in data.resource_metrics:
             if resource_metric.scope_metrics:
-                return resource_metric.scope_metrics[0].metrics
-        return None
+                for scope_metric in resource_metric.scope_metrics:
+                    for metric in scope_metric.metrics:
+                        metric_stub = self._extract_metric_from_opentelemetry(metric)
+                        if metric_stub:
+                            metric_stubs.append(metric_stub)
+
+        return tuple(metric_stubs) if metric_stubs else None
 
     def _clear_impl(self) -> None:
         self._span_exporter.clear()