feat: add support for Graphite metrics provider

Address issue #1403 by adding a Graphite
metrics provider.

Signed-off-by: Mike Ball <mikedball@gmail.com>

metricproviders/graphite/api.go

@@ -0,0 +1,156 @@
+package graphite
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"math"
+	"net/http"
+	"net/url"
+	"path"
+	"regexp"
+	"strconv"
+	"time"
+
+	log "github.com/sirupsen/logrus"
+)
+
+// API represents a Graphite API client
+type API interface {
+	Query(query string) (*float64, error)
+}
+
+// GraphiteAPI is a Graphite API client
+type APIClient struct {
+	url     url.URL
+	client  *http.Client
+	timeout time.Duration
+	logCTX  log.Entry
+}
+
+// Query performs a Graphite API query with the query it's passed
+func (api APIClient) Query(quer string) (*float64, error) {
+	query := api.trimQuery(quer)
+	u, err := url.Parse(fmt.Sprintf("./render?%s", query))
+	if err != nil {
+		return nil, err
+	}
+
+	q := u.Query()
+	q.Set("format", "json")
+	u.RawQuery = q.Encode()
+
+	u.Path = path.Join(api.url.Path, u.Path)
+	u = api.url.ResolveReference(u)
+
+	req, err := http.NewRequest("GET", u.String(), nil)
+	if err != nil {
+		return nil, err
+	}
+
+	ctx, cancel := context.WithTimeout(req.Context(), api.timeout)
+	defer cancel()
+
+	r, err := api.client.Do(req.WithContext(ctx))
+	if err != nil {
+		return nil, err
+	}
+	defer r.Body.Close()
+
+	b, err := ioutil.ReadAll(r.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	if 400 <= r.StatusCode {
+		return nil, fmt.Errorf("error response: %s", string(b))
+	}
+
+	var result graphiteResponse
+	err = json.Unmarshal(b, &result)
+	if err != nil {
+		return nil, err
+	}
+
+	var value *float64
+	for _, tr := range result {
+		for _, dp := range tr.DataPoints {
+			if dp.Value != nil {
+				value = dp.Value
+			}
+		}
+	}
+
+	return value, nil
+}
+
+func (api APIClient) trimQuery(q string) string {
+	space := regexp.MustCompile(`\s+`)
+	return space.ReplaceAllString(q, " ")
+}
+
+type graphiteDataPoint struct {
+	Value     *float64
+	TimeStamp time.Time
+}
+
+func (gdp *graphiteDataPoint) UnmarshalJSON(data []byte) error {
+	var v []interface{}
+	if err := json.Unmarshal(data, &v); err != nil {
+		return err
+	}
+
+	if len(v) != 2 {
+		return fmt.Errorf("error unmarshaling data point: %v", v)
+	}
+
+	switch v[0].(type) {
+	case nil:
+		// no value
+	case float64:
+		f, _ := v[0].(float64)
+		gdp.Value = &f
+	case string:
+		f, err := strconv.ParseFloat(v[0].(string), 64)
+		if err != nil {
+			return err
+		}
+		gdp.Value = &f
+	default:
+		f, ok := v[0].(float64)
+		if !ok {
+			return fmt.Errorf("error unmarshaling value: %v", v[0])
+		}
+		gdp.Value = &f
+	}
+
+	switch v[1].(type) {
+	case nil:
+		// no value
+	case float64:
+		ts := int64(math.Round(v[1].(float64)))
+		gdp.TimeStamp = time.Unix(ts, 0)
+	case string:
+		ts, err := strconv.ParseInt(v[1].(string), 10, 64)
+		if err != nil {
+			return err
+		}
+		gdp.TimeStamp = time.Unix(ts, 0)
+	default:
+		ts, ok := v[1].(int64)
+		if !ok {
+			return fmt.Errorf("error unmarshaling timestamp: %v", v[0])
+		}
+		gdp.TimeStamp = time.Unix(ts, 0)
+	}
+
+	return nil
+}
+
+type graphiteTargetResp struct {
+	Target     string              `json:"target"`
+	DataPoints []graphiteDataPoint `json:"datapoints"`
+}
+
+type graphiteResponse []graphiteTargetResp