loki returns 500 code for parse error in labels

[candlerb]

Describe the bug
If you provide a POST to /api/prom/push where the "labels" member is unparseable, loki rejects it with a 500 error. IMO it should be 400.

500 implies that (a) the fault is at the server side, not with the client-provided data; and (b) that it may work if the client resends the same request later, which clearly it won't in this case.

(Extracted from #927)

To Reproduce
Steps to reproduce the behavior:

1. Started loki (v0.3.0)
2. Submit with curl where "labels" contains an unterminated string - see end

Expected behavior
A 400 "Bad Request" error code (which is the behaviour if the request is invalid for other reasons, e.g. you submit a log timestamp that is too far in the past)

Environment:

• Ubuntu 18.04
• Manual build

Screenshots, promtail config, or terminal output

Trimmed request:

# curl -v -H Content-Type:application/json -d '{"streams":[{"labels":"{foo=\"bar}"}]}' http://127.0.0.1:3100/api/prom/push
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 3100 (#0)
> POST /api/prom/push HTTP/1.1
> Host: 127.0.0.1:3100
> User-Agent: curl/7.58.0
> Accept: */*
> Content-Type:application/json
> Content-Length: 38
>
* upload completely sent off: 38 out of 38 bytes
< HTTP/1.1 500 Internal Server Error
< Content-Type: text/plain; charset=utf-8
< X-Content-Type-Options: nosniff
< Date: Wed, 21 Aug 2019 10:29:58 GMT
< Content-Length: 53
<
parse error at line 1, col 6: literal not terminated
* Connection #0 to host 127.0.0.1 left intact

A more complete request which showed this:

# curl -v -H Content-Type:application/json -d '{"streams":[{"labels":"{job=\"fluent.info\",source=\"\",facility=\"\",severity=\"\",program=\"\",foo=\"\"\"}","entries":[{"ts":"2019-08-21T09:34:34.733352Z","line":"worker=\"0\" message=\"fluentd worker is now stopping worker=0\""}]}]}' http://127.0.0.1:3100/api/prom/push
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 3100 (#0)
> POST /api/prom/push HTTP/1.1
> Host: 127.0.0.1:3100
> User-Agent: curl/7.58.0
> Accept: */*
> Content-Type:application/json
> Content-Length: 235
>
* upload completely sent off: 235 out of 235 bytes
< HTTP/1.1 500 Internal Server Error
< Content-Type: text/plain; charset=utf-8
< X-Content-Type-Options: nosniff
< Date: Wed, 21 Aug 2019 10:24:12 GMT
< Content-Length: 54
<
parse error at line 1, col 71: literal not terminated
* Connection #0 to host 127.0.0.1 left intact