The /api/ds/query and /api/tsdb/query endpoints extract the intervalMs field from
each query in the request, but it currently seems to be ignored, at
least for queries to a Prometheus datasource. This changes
GetIntervalFrom function to check for the presence of intervalMs in the
query model if interval is missing.
(cherry picked from commit b7ea66b00d)
Co-authored-by: Ben Sully <ben.sully@grafana.com>