feat: add opentelemetry tracing (#215)

* testing first trace export

* instrumention client.py

* instrumenting job.py and adding documentation

* reconfiguring imports

* quick cleanup of unused variable

* adding more attributes in module and limiting complexity of instrumentation

* adding tests, nox and correct attribute additions in client & job

* adding tests, nox and correct attribute additions in client & job (left out of last commit)

* linting

* reformatting noxfile.[y

* addressing suggested changes

* adding suggested changes

* removing print statements

* setting same version across all OT [ackages and other reccommended changes

* suggested changes

* fixing packages issue in nox and updating documentation

* fixing module install issue

* restructuring design for testing adding first layer of tests (some still failing)

* adding reamining client tests and all job tests

* fixing linting issues

* fixing trace not defined issue

* fixing lint issues

* fixing documentation issues and python2 testing issue

* linting and fixing coverage issues

* adding suggested changes

* linting

* adding Shawn's suggested changes

* fixing _default_span_attribute_bug

* reverting uneccesxsary changes

* adding more tests for all job_ref parameters

* removing dependecny, ordering imports and other changes

* addressing Shawn concerns

* adding test and suggested changes

* adding opentelemetry to setup.py and other suggested changes

* adding reasoning for not adding to [all]

* linting

* adding Tim suggested changes

Co-authored-by: Tim Swast <swast@google.com>

README.rst

@@ -102,3 +102,41 @@ Perform a query
 
     for row in rows:
         print(row.name)
+
+Instrumenting With OpenTelemetry
+--------------------------------
+
+This application uses `OpenTelemetry`_ to output tracing data from
+API calls to BigQuery. To enable OpenTelemetry tracing in
+the BigQuery client the following PyPI packages need to be installed:
+
+.. _OpenTelemetry: https://opentelemetry.io
+
+.. code-block:: console
+
+    pip install google-cloud-bigquery[opentelemetry] opentelemetry-exporter-google-cloud
+
+After installation, OpenTelemetry can be used in the BigQuery
+client and in BigQuery jobs. First, however, an exporter must be
+specified for where the trace data will be outputted to. An
+example of this can be found here:
+
+.. code-block:: python
+
+    from opentelemetry import trace
+    from opentelemetry.sdk.trace import TracerProvider
+    from opentelemetry.sdk.trace.export import BatchExportSpanProcessor
+    from opentelemetry.exporter.cloud_trace import CloudTraceSpanExporter
+    trace.set_tracer_provider(TracerProvider())
+    trace.get_tracer_provider().add_span_processor(
+        BatchExportSpanProcessor(CloudTraceSpanExporter())
+    )
+
+In this example all tracing data will be published to the Google
+`Cloud Trace`_ console. For more information on OpenTelemetry, please consult the `OpenTelemetry documentation`_.
+
+.. _OpenTelemetry documentation: https://opentelemetry-python.readthedocs.io
+.. _Cloud Trace: https://cloud.google.com/trace
+
+
+