Prometheus: Introduce prometheus backend library (#83952)

* Move files to prometheus-library

* refactor core prometheus to use prometheus-library

* modify client transport options

* mock

* have a type

* import aliases

* rename

* call the right method

* remove unrelated test from the library

* update codeowners

* go work sync

* update go.work.sum

* make swagger-clean && make openapi3-gen

* add promlib to makefile

* remove clilogger

* Export the function

* update unit test

* add prometheus_test.go

* fix mock type

* use mapUtil from grafana-plugin-sdk-go

pkg/promlib/middleware/force_http_get.go

@@ -0,0 +1,30 @@
+package middleware
+
+import (
+	"net/http"
+
+	sdkhttpclient "github.com/grafana/grafana-plugin-sdk-go/backend/httpclient"
+
+	"github.com/grafana/grafana-plugin-sdk-go/backend/log"
+)
+
+func ForceHttpGet(logger log.Logger) sdkhttpclient.Middleware {
+	return sdkhttpclient.NamedMiddlewareFunc("force-http-get", func(opts sdkhttpclient.Options, next http.RoundTripper) http.RoundTripper {
+		// the prometheus library we use does not allow us to set the http method.
+		// it's behavior is to first try POST, and if it fails in certain ways
+		// (for example, by returning a method-not-allowed error), it will try GET.
+		// so here, we check if the http-method is POST, and if it is, we
+		// return an artificial method-not-allowed response.
+		// this will cause the prometheus library to retry with GET.
+		return sdkhttpclient.RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
+			if req.Method == http.MethodPost {
+				resp := &http.Response{
+					StatusCode: http.StatusMethodNotAllowed,
+				}
+				return resp, nil
+			}
+
+			return next.RoundTrip(req)
+		})
+	})
+}