Opentracing contrib: `DroppedSpan has no attribute transaction`

[JeckyTian]

Describe the bug: ...
When using Elastic APM with Opentracing, the program crashes on AttributeError: DroppedSpan object has no attribute transaction exception if span.finish() is called.
It turns out to be this line:

apm-agent-python/elasticapm/contrib/opentracing/span.py

Line 129 in 74cd1fa

self.elastic_apm_ref.transaction.end_span()

It will access transaction attribute which does NOT exist for DroppedSpan.

Expected behavior: ...
It should be expected to behave normally even if it's DroppedSpan.

Environment (please complete the following information)

• OS: [e.g. Linux] Linux
• Python version: 2.7
• Framework and version [e.g. Django 2.1]: N/A
• APM Server version: 7.4.0
• Agent version: 5.3.2

[beniwohli]

Great find @JeckyTian! I'll open a PR with a fix.

[JeckyTian]

@beniwohli Thanks, is there an expectation which release has this fix?

[beniwohli]

@JeckyTian we just published 5.3.3 of the agent which includes this bug fix. Thanks again for reporting!