legacy endpoint 400s metric queries

[owen-d]

This fix short-circuits metric queries to the legacy /api/prom/query endpoint. Previously it would return 5xx status codes and now it 400s upon seeing metric queries. As a small optimization it doesn't actually run the query anymore either.

• also fixes small bug where the /api/prom/query handler would not handle POST requests.
• closes /api/prom/query route incorrectly 500s when passed a metric query #1537

Examples

Allows regular log queries

$ curl -i http://localhost:3100/api/prom/query --data-urlencode 'query={job="jobba-the-hut"}'
HTTP/1.1 200 OK
Date: Fri, 17 Jan 2020 17:11:49 GMT
Content-Length: 15
Content-Type: text/plain; charset=utf-8

{"streams":[]}

Fails metric queries

$ curl -i http://localhost:3100/api/prom/query --data-urlencode 'query=sum(rate({job="jobba-the-hut"}[10m])) by (level)'
HTTP/1.1 400 Bad Request
Content-Type: text/plain; charset=utf-8
X-Content-Type-Options: nosniff
Date: Fri, 17 Jan 2020 17:09:32 GMT
Content-Length: 50

legacy endpoints only support streams result type

[cyriltovena]

LGTM

[cstyan]

👍