Add details about aggregation pipelines & avoid get(default) parameter

[angelini]

When issuing aggregations via Pymongo, they store the pipeline as a command attribute.

This PR adds support for storing the pipeline as a trace attribute.

We've also seen the following errors in production with a Mongo 3.6.12 instance an Pymongo 3.6.1.

Traceback (most recent call last): 
  File "<REDACTED>/.virtualenv/lib/python3.7/site-packages/pymongo-3.6.1-py3.7-linux-x86_64.egg/pymongo/monitoring.py", line 739, in publish_command_start 
    subscriber.started(event) 
  File "<REDACTED>/.virtualenv/lib/python3.7/site-packages/opencensus_ext_pymongo-0.1.2-py3.7.egg/opencensus/ext/pymongo/trace.py", line 54, in started 
    _attr = event.command.get(attr, default=None) 
TypeError: get() takes no keyword arguments 

It seems like not all event.command objects are SON instances (https://github.com/mongodb/mongo-python-driver/blob/master/bson/son.py#L32).

Since the default keyword argument isn't being used here, the default value is None, I opted to remove the use of it.

[reyang]

@tjzo please help to review.

[reyang]

Thanks for the improvement @angelini!
LGTM, let's wait for one day to see if any feedbacks from the original author @tjzo.

[c24t]

It seems like not all event.command objects are SON instances

It looks like this PR doesn't fix this? @angelini if you've got a sample command to reproduce the error we can open another ticket to track this.

[c24t]

Nit: might as well use self.command_attrs.get(key).

[angelini]

It seems like not all event.command objects are SON instances

It looks like this PR doesn't fix this?

As best I can tell the commands aren't SON objects, but they have a get method. This change has fixed it for our production use.

if you've got a sample command to reproduce the error we can open another ticket to track this.

Yeah that would be great, working on a getting one but so far we've only seen this in production and haven't been able to reproduce it locally.

[angelini]

@c24t I was just able to confirm from production that the command causing the error is an empty dictionary.

class DetectIncorrectCommand(pymongo.monitoring.CommandListener):
    def started(self, event):
        if not isinstance(event.command, bson.SON):
            print('command for (%s) %s' % (type(event.command), event.command))

Resulted in:

> command for (<class 'dict'>) {}

And this is the same error we were seeing.

In [1]: {}.get('a', 1)
Out[1]: 1

In [2]: {}.get('a', default=1)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-9332c12c9211> in <module>
----> 1 {}.get('a', default=1)

TypeError: get() takes no keyword arguments

Removing the use of the keyword arg (default=None) https://github.com/census-instrumentation/opencensus-python/pull/644/files#diff-bb0ccb334b607c4945ac2a1ed4ac9776R54 fixes it.

[reyang]

@angelini would you update the CHANGELOG.md and rebase to the latest master branch? Once done, we're ready to merge.

[angelini]

@reyang it's been rebased and the changelog has been updated

[reyang]

Thanks for your contribution! @angelini