chore: disable telemetry if otel endpoint isn't set (#3859)

# What does this PR do?

removes error:
ConnectionError: HTTPConnectionPool(host='localhost', port=4318): Max
retries exceeded with url: /v1/traces
(Caused by NewConnectionError('<urllib3.connection.HTTPConnection object
at 0x10fd98e60>: Failed to establish a
         new connection: [Errno 61] Connection refused'))


## Test Plan
uv run llama stack run starter
curl http://localhost:8321/v1/models
observe no error in server logs

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

@@ -4,6 +4,7 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+import os
 import threading
 from typing import Any
 
@@ -60,6 +61,7 @@ class TelemetryAdapter(Telemetry):
         # Recreating the telemetry adapter multiple times will result in duplicate span processors.
         # Since the library client can be recreated multiple times in a notebook,
         # the kernel will hold on to the span processor and cause duplicate spans to be written.
+        if os.environ.get("OTEL_EXPORTER_OTLP_ENDPOINT"):
             if _TRACER_PROVIDER is None:
                 provider = TracerProvider()
                 trace.set_tracer_provider(provider)
@@ -77,6 +79,8 @@ class TelemetryAdapter(Telemetry):
                 metric_reader = PeriodicExportingMetricReader(OTLPMetricExporter())
                 metric_provider = MeterProvider(metric_readers=[metric_reader])
                 metrics.set_meter_provider(metric_provider)
+        else:
+            logger.warning("OTEL_EXPORTER_OTLP_ENDPOINT is not set, skipping telemetry")
 
         self.meter = metrics.get_meter(__name__)
         self._lock = _global_lock