Preserve some /heartbeat traces in edxapp DD telemetry

[timmc-edx]

No description provided.

[robrap]

We may need a different percent of retention for LMS than other services. It should be very quick to just add a new retention rule that is edxapp (or LMS) specific, and tune it until we've got enough that are useful and not annoying.

They should all be appearing in the Live View, right? But I guess they wouldn't be recording traces.

[timmc-edx]

Oh... we actually are getting spans indexed for these: env:prod service:edx-edxapp-lms resource_name:"GET openedx.core.djangoapps.heartbeat.views.heartbeat" -- not sure why I wasn't seeing these before. I think I was using the wrong query, a URL-based one.

So maybe we can close this out? I think there was some other healthcheck-related thing we wanted to keep a small percentage of (access logs?) but I'm not sure.

[robrap]

I think we are excluding 99.9%, so we only have 0.1%. But, there should be some if we need them.

[timmc-edx]

Turns out the logs are present, but tagged service:edx-nginx. Filed #893 for that, and closing this one out.