api support for scalars

[owen-d]

What

• logcli prints scalars
• loghttp types defer to prometheus model types for json marshaling

loki response for instant query 1+1:

{
  "status": "success",
  "data": {
    "resultType": "scalar",
    "result": [
      1581712272.018,
      "2"
    ],
    "stats": {
      "summary": {
        "bytesProcessedPerSeconds": 0,
        "linesProcessedPerSeconds": 0,
        "totalBytesProcessed": 0,
        "totalLinesProcessed": 0,
        "execTime": 2.7652e-05
      },
      "store": {
        "totalChunksRef": 0,
        "totalChunksDownloaded": 0,
        "chunksDownloadTime": 0,
        "headChunkBytes": 0,
        "headChunkLines": 0,
        "decompressedBytes": 0,
        "decompressedLines": 0,
        "compressedBytes": 0,
        "totalDuplicates": 0
      },
      "ingester": {
        "totalReached": 0,
        "totalChunksMatched": 0,
        "totalBatches": 0,
        "totalLinesSent": 0,
        "headChunkBytes": 0,
        "headChunkLines": 0,
        "decompressedBytes": 0,
        "decompressedLines": 0,
        "compressedBytes": 0,
        "totalDuplicates": 0
      }
    }
  }
}

Prometheus response for the same

{
  "status": "success",
  "data": {
    "resultType": "scalar",
    "result": [
      1581711059,
      "2"
    ]
  }
}

[rfratto]

LGTM!

[codecov-io]

Codecov Report

Merging #1704 into master will decrease coverage by 0.19%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master    #1704     +/-   ##
=========================================
- Coverage   63.42%   63.23%   -0.2%     
=========================================
  Files         121      121             
  Lines        9003     9027     +24     
=========================================
- Hits         5710     5708      -2     
- Misses       2875     2899     +24     
- Partials      418      420      +2

Impacted Files	Coverage Δ
pkg/loghttp/query.go	41.66% <0%> (-3.34%)	⬇️
pkg/logql/marshal/query.go	72.61% <0%> (-8.72%)	⬇️
pkg/logcli/query/query.go	0% <0%> (ø)	⬆️
pkg/ingester/transfer.go	65% <0%> (-1.43%)	⬇️
pkg/promtail/targets/tailer.go	75.86% <0%> (ø)	⬆️