test: enable telemetry tests in server mode (#3927)

# What does this PR do?
- added a server-based test OLTP collector

## Test Plan
CI

tests/integration/telemetry/collectors/otlp.py

@@ -0,0 +1,117 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+"""OTLP HTTP telemetry collector used for server-mode tests."""
+
+import gzip
+import os
+import threading
+from http.server import BaseHTTPRequestHandler, HTTPServer
+from socketserver import ThreadingMixIn
+from typing import Any
+
+from opentelemetry.proto.collector.metrics.v1.metrics_service_pb2 import ExportMetricsServiceRequest
+from opentelemetry.proto.collector.trace.v1.trace_service_pb2 import ExportTraceServiceRequest
+
+from .base import BaseTelemetryCollector, SpanStub, attributes_to_dict, events_to_list
+
+
+class OtlpHttpTestCollector(BaseTelemetryCollector):
+    def __init__(self) -> None:
+        self._spans: list[SpanStub] = []
+        self._metrics: list[Any] = []
+        self._lock = threading.Lock()
+
+        class _ThreadingHTTPServer(ThreadingMixIn, HTTPServer):
+            daemon_threads = True
+            allow_reuse_address = True
+
+        configured_port = int(os.environ.get("LLAMA_STACK_TEST_COLLECTOR_PORT", "0"))
+
+        self._server = _ThreadingHTTPServer(("127.0.0.1", configured_port), _CollectorHandler)
+        self._server.collector = self  # type: ignore[attr-defined]
+        port = self._server.server_address[1]
+        self.endpoint = f"http://127.0.0.1:{port}"
+
+        self._thread = threading.Thread(target=self._server.serve_forever, name="otel-test-collector", daemon=True)
+        self._thread.start()
+
+    def _handle_traces(self, request: ExportTraceServiceRequest) -> None:
+        new_spans: list[SpanStub] = []
+
+        for resource_spans in request.resource_spans:
+            resource_attrs = attributes_to_dict(resource_spans.resource.attributes)
+
+            for scope_spans in resource_spans.scope_spans:
+                for span in scope_spans.spans:
+                    attributes = attributes_to_dict(span.attributes)
+                    events = events_to_list(span.events) if span.events else None
+                    trace_id = span.trace_id.hex() if span.trace_id else None
+                    span_id = span.span_id.hex() if span.span_id else None
+                    new_spans.append(SpanStub(span.name, attributes, resource_attrs or None, events, trace_id, span_id))
+
+        if not new_spans:
+            return
+
+        with self._lock:
+            self._spans.extend(new_spans)
+
+    def _handle_metrics(self, request: ExportMetricsServiceRequest) -> None:
+        new_metrics: list[Any] = []
+        for resource_metrics in request.resource_metrics:
+            for scope_metrics in resource_metrics.scope_metrics:
+                new_metrics.extend(scope_metrics.metrics)
+
+        if not new_metrics:
+            return
+
+        with self._lock:
+            self._metrics.extend(new_metrics)
+
+    def _snapshot_spans(self) -> tuple[SpanStub, ...]:
+        with self._lock:
+            return tuple(self._spans)
+
+    def _snapshot_metrics(self) -> Any | None:
+        with self._lock:
+            return list(self._metrics) if self._metrics else None
+
+    def _clear_impl(self) -> None:
+        with self._lock:
+            self._spans.clear()
+            self._metrics.clear()
+
+    def shutdown(self) -> None:
+        self._server.shutdown()
+        self._server.server_close()
+        self._thread.join(timeout=1)
+
+
+class _CollectorHandler(BaseHTTPRequestHandler):
+    def do_POST(self) -> None:  # noqa: N802 Function name `do_POST` should be lowercase
+        collector: OtlpHttpTestCollector = self.server.collector  # type: ignore[attr-defined]
+        length = int(self.headers.get("content-length", "0"))
+        body = self.rfile.read(length)
+        if self.headers.get("content-encoding") == "gzip":
+            body = gzip.decompress(body)
+
+        if self.path == "/v1/traces":
+            request = ExportTraceServiceRequest()
+            request.ParseFromString(body)
+            collector._handle_traces(request)
+            self._respond_ok()
+        elif self.path == "/v1/metrics":
+            request = ExportMetricsServiceRequest()
+            request.ParseFromString(body)
+            collector._handle_metrics(request)
+            self._respond_ok()
+        else:
+            self.send_response(404)
+            self.end_headers()
+
+    def _respond_ok(self) -> None:
+        self.send_response(200)
+        self.end_headers()