tracing: trace_call returns None if opentelemetry is not enabled or if no session but that makes it awkward to seamlessly use library, it should return a NoopSpan

If we examine the code for trace_call it returns None

python-spanner/google/cloud/spanner_v1/_opentelemetry_tracing.py

Lines 32 to 36 in a941adb

    
           def trace_call(name, session, extra_attributes=None): 
        
               if not HAS_OPENTELEMETRY_INSTALLED or not session: 
        
                   # Empty context manager. Users will have to check if the generated value is None or a span 
        
                   yield None 
        
                   return

and when we try to perform what should be a simple thing like add annotations, we have to awkwardly check if span is not None and that's not a good user experience.

Remedy

Let's implement a NoopSpan which has all the methods for a Span but does nothing

	def trace_call(name, session, extra_attributes=None):
	if not HAS_OPENTELEMETRY_INSTALLED or not session:
	# Empty context manager. Users will have to check if the generated value is None or a span
	yield None
	return