Fix telemetry singleton pattern to respect dynamic environment variables

- Modified Telemetry.__init__ to prevent re-initialization with _initialized flag
- Updated _safe_telemetry_operation to check _is_telemetry_disabled() dynamically
- Added comprehensive tests for environment variables set after singleton creation
- Fixed singleton contamination in existing tests by adding proper reset
- Resolves issue #2945 where CREWAI_DISABLE_TELEMETRY=true was ignored when set after import

Co-Authored-By: João <joao@crewai.com>

tests/telemetry/test_telemetry.py

@@ -22,6 +22,7 @@ from opentelemetry import trace
 )
 def test_telemetry_environment_variables(env_var, value, expected_ready):
     """Test telemetry state with different environment variable configurations."""
+    Telemetry._instance = None
     with patch.dict(os.environ, {env_var: value}):
         with patch("crewai.telemetry.telemetry.TracerProvider"):
             telemetry = Telemetry()
@@ -30,6 +31,7 @@ def test_telemetry_environment_variables(env_var, value, expected_ready):
 
 def test_telemetry_enabled_by_default():
     """Test that telemetry is enabled by default."""
+    Telemetry._instance = None
     with patch.dict(os.environ, {}, clear=True):
         with patch("crewai.telemetry.telemetry.TracerProvider"):
             telemetry = Telemetry()