Timeout if populating iterators takes too long (#4291)
Timeout if populating iterators takes too long (#4291)

Right now promql won't time out a request if populating the iterators
takes a long time.

Signed-off-by: Thomas Jackson <2d00bc2c74768cf27c5a78623e81f1b7f1ca2826@gmail.com>

Fixes #4289
GoogleCloudPlatform/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,keegancsmith/prometheus,prometheus/prometheus,prometheus/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,kangwoo/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,prometheus/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,kangwoo/prometheus,prometheus/prometheus,alileza/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,kangwoo/prometheus,alileza/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus