query: set default times for query_exemplars API

[paulgrav]

Signed-off-by: Paul Grave paul@stomer.net

Changes

Fixes #5011

I replaced calls to cortexutil.ParseTime with parseTimeParam in NewExemplarsHandler since the later allows default times to be set.
I removed the cortexutil since the above change made it redundant.

The impact is that this behaviour mirrors Prometheus’ behaviour and allows exemplars to be displayed in Grafana.

Verification

I effected some functional tests. I ran my own container (paulgrave/thanos:0.24.1) on my cluster and validated that exemplars are now displayed in Grafana.
Executed curl -v -d query=test http://localhost:10902/api/v1/query_exemplars to validate that a) API doesn’t error b) returns a 200 instead of a 400.
Executed curl -v -d query='my_requests_total{app="prom-py"}' -d start=1640770731.689 -d end=(date +%s) http://localhost:10902/api/v1/query_exemplars | jq to validate that exemplars are indeed returned like before.
Thanos is running as expected on my local cluster.

Unfortunately make lint and make test are broken on my M1 Mac.

[yeya24]

LGTM. Thanks for your contribution!