Deflake OpenTelemetry assertion in system tests

[larkee]

The OpenTelemetry assertions in the system tests fail if the transaction is aborted and retried. This is extremely brittle so the assertions need to be updated to account for retrying aborted transactions.

Example of test failure:

________ TestSessionAPI.test_transaction_read_and_insert_then_rollback _________

self = <tests.system.test_system.TestSessionAPI testMethod=test_transaction_read_and_insert_then_rollback>

    @RetryErrors(exception=exceptions.ServerError)
    @RetryErrors(exception=exceptions.Aborted)
    def test_transaction_read_and_insert_then_rollback(self):
        retry = RetryInstanceState(_has_all_ddl)
        retry(self._db.reload)()

        session = self._db.session()
        session.create()
        self.to_delete.append(session)

        with self._db.batch() as batch:
            batch.delete(self.TABLE, self.ALL)

        transaction = session.transaction()
        transaction.begin()

        rows = list(transaction.read(self.TABLE, self.COLUMNS, self.ALL))
        self.assertEqual(rows, [])

        transaction.insert(self.TABLE, self.COLUMNS, self.ROW_DATA)

        # Inserted rows can't be read until after commit.
        rows = list(transaction.read(self.TABLE, self.COLUMNS, self.ALL))
        self.assertEqual(rows, [])
        transaction.rollback()

        rows = list(session.read(self.TABLE, self.COLUMNS, self.ALL))
        self.assertEqual(rows, [])

        if HAS_OPENTELEMETRY_INSTALLED:
            span_list = self.memory_exporter.get_finished_spans()
>           self.assertEqual(len(span_list), 8)
E           AssertionError: 14 != 8

tests/system/test_system.py:1026: AssertionError
----------------------------- Captured stdout call -----------------------------
409 Transaction was aborted., Trying again in 1 seconds...
------------------------------ Captured log call -------------------------------
WARNING  opentelemetry.trace:__init__.py:468 Overriding current TracerProvider
WARNING  opentelemetry.trace:__init__.py:468 Overriding current TracerProvider