add support for max_traces_per_second option to datadog.conf

[joshuabaird]

What does this PR do?

This PR does two things:

• Uncomments the [trace.sampler] section in the stock datadog.conf
• Adds the max_traces_per_second configuration option to datadog.conf

This allows a user to configure this value by setting the MAX_TRACES_PER_SECOND environment variable.

Motivation

We are heavy users of APM. We require a higher max_traces_per_second than what the default provides (10).

What inspired you to submit this pull request?

The inability to configure max_traces_per_second via an environment variable.

[joshuabaird]

Can someone please take a look at this? Anything that needs to be done before it's approved/merged?

[xvello]

hi @joshuabaird ,

Sorry for the delay. The option name is indeed specific enough that prefixing it with trace_sampler is not necessary.
I just have one comment about how you handled the missing trace.sampler section, what do you think?

[xvello]

Instead of adding the section here, what do you think about patching set_property in config_builder.py to do

if not self.config.has_section(section):
    self.config.add_section(section)

See https://docs.python.org/2/library/configparser.html#ConfigParser.RawConfigParser.add_section

I do think failing in config_builder.py because the section does not exist is a bug.

[joshuabaird]

Good idea - much better than my original hack. Done!