Fix v1 label API to be Prometheus-compatible (#1322)

The label and label values API will now return the expected form
of:

```json
{
  "status": "success",
  "data": [
    "label1",
    "label2",
    "labeln"
  ]
}
```

docs/api.md

@@ -14,6 +14,8 @@ The HTTP API includes the following endpoints:
 - [`POST /loki/api/v1/push`](#post-lokiapiv1push)
 - [`GET /api/prom/tail`](#get-apipromtail)
 - [`GET /api/prom/query`](#get-apipromquery)
+- [`GET /api/prom/label`](#get-apipromlabel)
+- [`GET /api/prom/label/<name>/values`](#get-apipromlabelnamevalues)
 - [`POST /api/prom/push`](#post-apiprompush)
 - [`GET /ready`](#get-ready)
 - [`POST /flush`](#post-flush)
@@ -38,6 +40,8 @@ These endpoints are exposed by just the querier:
 - [`GET /loki/api/v1/tail`](#get-lokiapiv1tail)
 - [`GET /api/prom/tail`](#get-lokiapipromtail)
 - [`GET /api/prom/query`](#get-apipromquery)
+- [`GET /api/prom/label`](#get-apipromlabel)
+- [`GET /api/prom/label/<name>/values`](#get-apipromlabelnamevalues)
 
 While these endpoints are exposed by just the distributor:
 
@@ -348,7 +352,8 @@ Response:
 
 ```
 {
-  "values": [
+  "status": "success",
+  "data": [
     <label string>,
     ...
   ]
@@ -360,7 +365,8 @@ Response:
 ```bash
 $ curl -G -s  "http://localhost:3100/loki/api/v1/label" | jq
 {
-  "values": [
+  "status": "success",
+  "data": [
     "foo",
     "bar",
     "baz"
@@ -383,7 +389,8 @@ Response:
 
 ```
 {
-  "values": [
+  "status": "success",
+  "data": [
     <label value>,
     ...
   ]
@@ -395,7 +402,8 @@ Response:
 ```bash
 $ curl -G -s  "http://localhost:3100/loki/api/v1/label/foo/values" | jq
 {
-  "values": [
+  "status": "success",
+  "data": [
     "cat",
     "dog",
     "axolotl"
@@ -597,11 +605,77 @@ $ curl -G -s  "http://localhost:3100/api/prom/query" --data-urlencode '{foo="bar
 }
 ```
 
+## `GET /api/prom/label`
+
+> **WARNING**: `/api/prom/label` is DEPRECATED; use `/loki/api/v1/label`
+
+`/api/prom/label` retrieves the list of known labels within a given time span. It
+accepts the following query parameters in the URL:
+
+- `start`: The start time for the query as a nanosecond Unix epoch. Defaults to 6 hours ago.
+- `end`: The start time for the query as a nanosecond Unix epoch. Defaults to now.
+
+In microservices mode, `/api/prom/label` is exposed by the querier.
+
+Response:
+
+```
+{
+  "values": [
+    <label string>,
+    ...
+  ]
+}
+```
+
 ### Examples
 
 ```bash
-$ curl -H "Content-Type: application/json" -XPOST -s "https://localhost:3100/loki/api/v1/push" --data-raw \
-  '{"streams": [{ "labels": "{foo=\"bar\"}", "entries": [{ "ts": "2018-12-18T08:28:06.801064-04:00", "line": "fizzbuzz" }] }]}'
+$ curl -G -s  "http://localhost:3100/api/prom/label" | jq
+{
+  "values": [
+    "foo",
+    "bar",
+    "baz"
+  ]
+}
+```
+
+## `GET /api/prom/label/<name>/values`
+
+> **WARNING**: `/api/prom/label/<name>/values` is DEPRECATED; use `/loki/api/v1/label/<name>/values`
+
+`/api/prom/label/<name>/values` retrieves the list of known values for a given
+label within a given time span. It accepts the following query parameters in
+the URL:
+
+- `start`: The start time for the query as a nanosecond Unix epoch. Defaults to 6 hours ago.
+- `end`: The start time for the query as a nanosecond Unix epoch. Defaults to now.
+
+In microservices mode, `/api/prom/label/<name>/values` is exposed by the querier.
+
+Response:
+
+```
+{
+  "values": [
+    <label value>,
+    ...
+  ]
+}
+```
+
+### Examples
+
+```bash
+$ curl -G -s  "http://localhost:3100/api/prom/label/foo/values" | jq
+{
+  "values": [
+    "cat",
+    "dog",
+    "axolotl"
+  ]
+}
 ```
 
 ## `POST /api/prom/push`

pkg/logcli/labelquery/labels.go

@@ -35,5 +35,5 @@ func (q *LabelQuery) ListLabels(c *client.Client) []string {
 	if err != nil {
 		log.Fatalf("Error doing request: %+v", err)
 	}
-	return labelResponse.Values
+	return labelResponse.Data
 }

pkg/loghttp/labels.go

@@ -12,7 +12,8 @@ import (
 
 // LabelResponse represents the http json response to a label query
 type LabelResponse struct {
-	Values []string `json:"values,omitempty"`
+	Status string   `json:"status"`
+	Data   []string `json:"data,omitempty"`
 }
 
 // LabelSet is a key/value pair mapping of labels

pkg/logql/marshal/marshal.go

@@ -1,5 +1,5 @@
-// Package marshal converts internal objects to loghttp model objects.  This package is designed to work with
-//  models in pkg/loghttp.
+// Package marshal converts internal objects to loghttp model objects.  This
+// package is designed to work with models in pkg/loghttp.
 package marshal
 
 import (
@@ -14,7 +14,8 @@ import (
 	"github.com/prometheus/prometheus/promql"
 )
 
-// WriteQueryResponseJSON marshals the promql.Value to v1 loghttp JSON and then writes it to the provided io.Writer
+// WriteQueryResponseJSON marshals the promql.Value to v1 loghttp JSON and then
+// writes it to the provided io.Writer.
 func WriteQueryResponseJSON(v promql.Value, w io.Writer) error {
 
 	value, err := NewResultValue(v)
@@ -34,14 +35,19 @@ func WriteQueryResponseJSON(v promql.Value, w io.Writer) error {
 	return json.NewEncoder(w).Encode(q)
 }
 
-// WriteLabelResponseJSON marshals a logproto.LabelResponse to v1 loghttp JSON and then writes it to the provided io.Writer
-//  Note that it simply directly marshals the value passed in.  This is because the label currently marshals
-//  cleanly to the v1 http protocol.  If this ever changes, it will be caught by testing.
+// WriteLabelResponseJSON marshals a logproto.LabelResponse to v1 loghttp JSON
+// and then writes it to the provided io.Writer.
 func WriteLabelResponseJSON(l logproto.LabelResponse, w io.Writer) error {
-	return json.NewEncoder(w).Encode(l)
+	v1Response := loghttp.LabelResponse{
+		Status: "success",
+		Data:   l.GetValues(),
+	}
+
+	return json.NewEncoder(w).Encode(v1Response)
 }
 
-// WriteTailResponseJSON marshals the legacy.TailResponse to v1 loghttp JSON and then writes it to the provided connection
+// WriteTailResponseJSON marshals the legacy.TailResponse to v1 loghttp JSON and
+// then writes it to the provided connection.
 func WriteTailResponseJSON(r legacy.TailResponse, c *websocket.Conn) error {
 	v1Response, err := NewTailResponse(r)
 

pkg/logql/marshal/marshal_test.go

@@ -212,7 +212,7 @@ var labelTests = []struct {
 				"value",
 			},
 		},
-		`{"values": ["label1", "test", "value"]}`,
+		`{"status": "success", "data": ["label1", "test", "value"]}`,
 	},
 }