Add distributor endpoint for validating PromQL expressions

[juliusv]

Update: Now the response looks like this:

In case of a good expression:

HTTP status code: 200 OK.

{
  "status": "success"
}

In case of an error:

HTTP status code: 400 Bad Request.

{
  "error": "parse error at line 1, char 4: could not parse remaining input \",expr\"...",
  "errorType": "bad_data",
  "location": {
    "line": 1,
    "pos": 4
  },
  "status": "error"
}

Fixes #176

[juliusv]

@davkal Is this sufficient, or would you like the parser error message in case of an invalid expression as well?

[davkal]

@juliusv The error would be great. But I'd like to merge this first.

[juliusv]

Now the response looks like this:

In case of a good expression:

HTTP status code: 200 OK.

{
  "status": "success"
}

In case of an error:

HTTP status code: 400 Bad Request.

{
  "error": "parse error at line 1, char 4: could not parse remaining input \",expr\"...",
  "errorType": "bad_data",
  "location": {
    "line": 1,
    "pos": 4
  },
  "status": "error"
}

[tomwilkie]

LGTM!