ZO-1629: Work around NonRecordingSpan opentelemetry bug

AFAICT otel violates its own API here, should use get_span_context() instead:
  File "opentelemetry/instrumentation/utils.py", line 160, in _generate_opentelemetry_traceparent
    _span_id = trace.format_span_id(span.context.span_id)
  AttributeError: 'NonRecordingSpan' object has no attribute 'context'

I'm backporting open-telemetry/opentelemetry-python-contrib#1200
which sidesteps the issue by using propagate to generate the traceparent header
instead of doing it manually.

core/docs/changelog/ZO-1629.change

@@ -0,0 +1 @@
+ZO-1629: Work around NonRecordingSpan opentelemetry bug

core/src/zeit/connector/postgresql.py

@@ -4,6 +4,7 @@
 from google.cloud.storage.retry import DEFAULT_RETRY
 from io import BytesIO, StringIO
 from logging import getLogger
+from opentelemetry.trace.propagation import tracecontext
 from operator import itemgetter
 from sqlalchemy import Boolean, TIMESTAMP, Unicode, UnicodeText
 from sqlalchemy import Column, ForeignKey, select
@@ -18,6 +19,7 @@
 from zeit.connector.resource import CachedResource
 import collections
 import opentelemetry.instrumentation.sqlalchemy
+import opentelemetry.instrumentation.utils
 import os
 import os.path
 import sqlalchemy
@@ -66,8 +68,7 @@ def __init__(self, dsn, storage_project, storage_bucket,
         self.session = sqlalchemy.orm.scoped_session(
             sqlalchemy.orm.sessionmaker(bind=self.engine, future=True))
         zope.sqlalchemy.register(self.session)
-        EngineTracer(zeit.cms.tracing.default_tracer(),
-                     self.engine, enable_commenter=True)
+        EngineTracer(zeit.cms.tracing.default_tracer(), self.engine)
         self.gcs_client = storage.Client(project=storage_project)
         self.bucket = self.gcs_client.bucket(storage_bucket)
 
@@ -448,4 +449,15 @@ def _before_cur_exec(
         for k, v in params.items():
             p.write('%s=%r\n' % (k, str(v)[:100]))
         context._otel_span.set_attribute('db.parameters', p.getvalue())
+        # XXX We bypass enable_commenter in superclass, until
+        # opentelemetry-python-contrib#1200 is released.
+        statement += self._generate_comment()
         return statement, params
+
+    propagator = tracecontext.TraceContextTextMapPropagator()
+
+    def _generate_comment(self):
+        meta = {}
+        self.propagator.inject(meta)
+        return opentelemetry.instrumentation.utils._generate_sql_comment(
+            **meta)