fix: make telemetry optional for agents (#3705)

# What does this PR do?

there is a lot of code in the agents API using the telemetry API and its
helpers without checking if that API is even enabled.

This is the only API besides inference actively using telemetry code, so
after this telemetry can be optional for the entire stack


resolves #3665


## Test Plan

existing agent tests.

Signed-off-by: Charlie Doern <cdoern@redhat.com>

llama_stack/providers/registry/agents.py

@@ -36,6 +36,9 @@ def available_providers() -> list[ProviderSpec]:
                 Api.tool_runtime,
                 Api.tool_groups,
             ],
+            optional_api_dependencies=[
+                Api.telemetry,
+            ],
             description="Meta's reference implementation of an agent system that can use tools, access vector databases, and perform complex reasoning tasks.",
         ),
     ]