m3query doesn't filter labels properly using the != operator

[pavelnikolov]

M3Query fails to filter our results when using the not equal operator with labels (i.e. != )
The query below:

sum(up{some_label!=""}) by (job)

works fine in Prometheus but consistently fails in M3Query...
Just a little bit more context - the some_label only exists on some of our metrics. M3Query returns every single target (i.e. up series) from M3DB instead of just returning the targets that contain the specified label.

1. What service is experiencing the issue?
   M3Query

2. What is the configuration of the service? Please include any YAML files, as well as namespace / placement configuration (with any sensitive information anonymized if necessary).
   n/a

3. How are you using the service? For example, are you performing read/writes to the service via Prometheus, or are you using a custom script?
   Prometheus -> M3Coordinator -> M3DB, then M3Query reads from M3DB

4. Is there a reliable way to reproduce the behavior? If so, please provide detailed instructions.
   I'd be happy to jump on a call and demonstrate this. I haven't had the time to build an example or to write a unit test that fails.

[robskillington]

Interesting, I think this will actually be fixed when this PR is merged:
#1986

cc @arnikola

[pavelnikolov]

Yes, it's resolved