Add --prometheus-verb to support POST requests to prometheus servers

cmd/adapter/adapter.go

@@ -74,6 +74,8 @@ type PrometheusAdapter struct {
 	PrometheusTokenFile string
 	// PrometheusHeaders is a k=v list of headers to set on requests to PrometheusURL
 	PrometheusHeaders []string
+	// PrometheusVerb is a verb to set on requests to PrometheusURL
+	PrometheusVerb string
 	// AdapterConfigFile points to the file containing the metrics discovery configuration.
 	AdapterConfigFile string
 	// MetricsRelistInterval is the interval at which to relist the set of available metrics
@@ -90,6 +92,10 @@ func (cmd *PrometheusAdapter) makePromClient() (prom.Client, error) {
 		return nil, fmt.Errorf("invalid Prometheus URL %q: %v", baseURL, err)
 	}
 
+	if cmd.PrometheusVerb != http.MethodGet && cmd.PrometheusVerb != http.MethodPost {
+		return nil, fmt.Errorf("unsupported Prometheus Http Verb %q", cmd.PrometheusVerb)
+	}
+
 	var httpClient *http.Client
 
 	if cmd.PrometheusCAFile != "" {
@@ -117,7 +123,7 @@ func (cmd *PrometheusAdapter) makePromClient() (prom.Client, error) {
 	}
 	genericPromClient := prom.NewGenericAPIClient(httpClient, baseURL, parseHeaderArgs(cmd.PrometheusHeaders))
 	instrumentedGenericPromClient := mprom.InstrumentGenericAPIClient(genericPromClient, baseURL.String())
-	return prom.NewClientForAPI(instrumentedGenericPromClient), nil
+	return prom.NewClientForAPI(instrumentedGenericPromClient, cmd.PrometheusVerb), nil
 }
 
 func (cmd *PrometheusAdapter) addFlags() {
@@ -137,6 +143,8 @@ func (cmd *PrometheusAdapter) addFlags() {
 		"Optional file containing the bearer token to use when connecting with Prometheus")
 	cmd.Flags().StringArrayVar(&cmd.PrometheusHeaders, "prometheus-header", cmd.PrometheusHeaders,
 		"Optional header to set on requests to prometheus-url. Can be repeated")
+	cmd.Flags().StringVar(&cmd.PrometheusVerb, "prometheus-verb", cmd.PrometheusVerb,
+		"HTTP Verb to set on requests to Prometheus.")
 	cmd.Flags().StringVar(&cmd.AdapterConfigFile, "config", cmd.AdapterConfigFile,
 		"Configuration file containing details of how to transform between Prometheus metrics "+
 			"and custom metrics API resources")
@@ -274,6 +282,7 @@ func main() {
 	// set up flags
 	cmd := &PrometheusAdapter{
 		PrometheusURL:         "https://localhost",
+		PrometheusVerb:        http.MethodGet,
 		MetricsRelistInterval: 10 * time.Minute,
 	}
 	cmd.Name = "prometheus-metrics-adapter"

pkg/client/api.go

@@ -25,6 +25,7 @@ import (
 	"net/http"
 	"net/url"
 	"path"
+	"strings"
 	"time"
 
 	"github.com/prometheus/common/model"
@@ -53,13 +54,26 @@ type httpAPIClient struct {
 func (c *httpAPIClient) Do(ctx context.Context, verb, endpoint string, query url.Values) (APIResponse, error) {
 	u := *c.baseURL
 	u.Path = path.Join(c.baseURL.Path, endpoint)
+	var reqBody io.Reader
+	if verb == http.MethodGet {
 		u.RawQuery = query.Encode()
-	req, err := http.NewRequest(verb, u.String(), nil)
+	} else if verb == http.MethodPost {
+		reqBody = strings.NewReader(query.Encode())
+	}
+
+	req, err := http.NewRequest(verb, u.String(), reqBody)
 	if err != nil {
 		return APIResponse{}, fmt.Errorf("error constructing HTTP request to Prometheus: %v", err)
 	}
 	req.WithContext(ctx)
-	req.Header = c.headers
+	for key, values := range c.headers {
+		for _, value := range values {
+			req.Header.Add(key, value)
+		}
+	}
+	if verb == http.MethodPost {
+		req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	}
 
 	resp, err := c.client.Do(req)
 	defer func() {
@@ -132,19 +146,21 @@ const (
 // queryClient is a Client that connects to the Prometheus HTTP API.
 type queryClient struct {
 	api  GenericAPIClient
+	verb string
 }
 
 // NewClientForAPI creates a Client for the given generic Prometheus API client.
-func NewClientForAPI(client GenericAPIClient) Client {
+func NewClientForAPI(client GenericAPIClient, verb string) Client {
 	return &queryClient{
 		api:  client,
+		verb: verb,
 	}
 }
 
 // NewClient creates a Client for the given HTTP client and base URL (the location of the Prometheus server).
-func NewClient(client *http.Client, baseURL *url.URL, headers http.Header) Client {
+func NewClient(client *http.Client, baseURL *url.URL, headers http.Header, verb string) Client {
 	genericClient := NewGenericAPIClient(client, baseURL, headers)
-	return NewClientForAPI(genericClient)
+	return NewClientForAPI(genericClient, verb)
 }
 
 func (h *queryClient) Series(ctx context.Context, interval model.Interval, selectors ...Selector) ([]Series, error) {
@@ -160,7 +176,7 @@ func (h *queryClient) Series(ctx context.Context, interval model.Interval, selec
 		vals.Add("match[]", string(selector))
 	}
 
-	res, err := h.api.Do(ctx, "GET", seriesURL, vals)
+	res, err := h.api.Do(ctx, h.verb, seriesURL, vals)
 	if err != nil {
 		return nil, err
 	}
@@ -180,7 +196,7 @@ func (h *queryClient) Query(ctx context.Context, t model.Time, query Selector) (
 		vals.Set("timeout", model.Duration(timeout).String())
 	}
 
-	res, err := h.api.Do(ctx, "GET", queryURL, vals)
+	res, err := h.api.Do(ctx, h.verb, queryURL, vals)
 	if err != nil {
 		return QueryResult{}, err
 	}
@@ -207,7 +223,7 @@ func (h *queryClient) QueryRange(ctx context.Context, r Range, query Selector) (
 		vals.Set("timeout", model.Duration(timeout).String())
 	}
 
-	res, err := h.api.Do(ctx, "GET", queryRangeURL, vals)
+	res, err := h.api.Do(ctx, h.verb, queryRangeURL, vals)
 	if err != nil {
 		return QueryResult{}, err
 	}