Use Content-Type data for telemetry versioning

ProcessorForRequestHeader now looks first for a header like
`Content-Type: application/json; schema="prometheus/telemetry";
version="0.0.1"` before falling back to checking
`X-Prometheus-API-Version`.

retrieval/format/discriminator.go

@@ -15,6 +15,7 @@ package format
 
 import (
 	"fmt"
+	"mime"
 	"net/http"
 )
 
@@ -39,7 +40,25 @@ func (r *registry) ProcessorForRequestHeader(header http.Header) (processor Proc
 		return
 	}
 
-	prometheusApiVersion := header.Get("X-Prometheus-API-Version")
+	mediatype, params, err := mime.ParseMediaType(header.Get("Content-Type"))
+
+	if err != nil {
+		err = fmt.Errorf("Invalid Content-Type header %q: %s", header.Get("Content-Type"), err)
+		return
+	}
+
+	if mediatype != "application/json" {
+		err = fmt.Errorf("Unsupported media type %q, expected %q", mediatype, "application/json")
+		return
+	}
+
+	var prometheusApiVersion string
+
+	if params["schema"] == "prometheus/telemetry" && params["version"] != "" {
+		prometheusApiVersion = params["version"]
+	} else {
+		prometheusApiVersion = header.Get("X-Prometheus-API-Version")
+	}
 
 	switch prometheusApiVersion {
 	case "0.0.1":

retrieval/format/discriminator_test.go

@@ -31,12 +31,22 @@ func testDiscriminatorHttpHeader(t test.Tester) {
 			err:    fmt.Errorf("Received illegal and nil header."),
 		},
 		{
-			input:  map[string]string{"X-Prometheus-API-Version": "0.0.0"},
+			input:  map[string]string{"Content-Type": "application/json", "X-Prometheus-API-Version": "0.0.0"},
 			output: nil,
 			err:    fmt.Errorf("Unrecognized API version 0.0.0"),
 		},
 		{
-			input:  map[string]string{"X-Prometheus-API-Version": "0.0.1"},
+			input:  map[string]string{"Content-Type": "application/json", "X-Prometheus-API-Version": "0.0.1"},
+			output: Processor001,
+			err:    nil,
+		},
+		{
+			input:  map[string]string{"Content-Type": `application/json; schema="prometheus/telemetry"; version=0.0.0`},
+			output: nil,
+			err:    fmt.Errorf("Unrecognized API version 0.0.0"),
+		},
+		{
+			input:  map[string]string{"Content-Type": `application/json; schema="prometheus/telemetry"; version=0.0.1`},
 			output: Processor001,
 			err:    nil,
 		},