observability: annotate Session+SessionPool events

[odeke-em]

This change adds annotations for session and session pool events to aid customers in debugging latency issues with session pool malevolence and also for maintainers to figure out which session pool type is the most appropriate.

Updates #1170

[odeke-em]

Exhibit

On running examples/grpc_instrumentation_enabled.py we get back these traces

[odeke-em]

cc @harshachinta

[odeke-em]

Kindly cc-ing you @harshachinta.

[harshachinta]

Why is this not "Acquiring a session"? Lets maintain span consistency across all the pools.

[odeke-em]

Because really it is waiting for a session to become available per

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

Lines 218 to 238 in b2e6762

	def get(self, timeout=None):
	"""Check a session out from the pool.
	:type timeout: int
	:param timeout: seconds to block waiting for an available session
	:rtype: :class:`~google.cloud.spanner_v1.session.Session`
	:returns: an existing session from the pool, or a newly-created
	session.
	:raises: :exc:`queue.Empty` if the queue is empty.
	"""
	if timeout is None:
	timeout = self.default_timeout
	session = self._sessions.get(block=True, timeout=timeout)
	if not session.exists():
	session = self._database.session()
	session.create()
	return session

and in our document requirements there is a clear need to add that case

[odeke-em]

Kindly please take another look @harshachinta, feedback addressed, thank you for the code review!

[harshachinta]

current_span is not initialized.

[odeke-em]

Sheesh, sorry I had a bunch of benchmarks running locally and they interrupted my nox -s unit-3.8 tests so didn't see those but all fixed now.

[odeke-em]

@harshachinta @surbhigarg92 @sakthivelmanii kindly help me trigger the bots for this PR too. Thank you.

[odeke-em]

This PR could use some bots runs. Thank you!

[odeke-em]

This PR could use some bots runs. Thank you!