feat(prometheus): Add minimum range interval based on scrape interval

Inspired by how Grafana does `$__rate_interval`, this adds the feature
that the minimum range passed to Prometheus will be 4x the configured
scrape interval. This allows users of 1 minutes scrape intervals to see
metrics on the 30 minute and 1 hour time range, which is currently
impossible.

For details of Grafana's implementation:
https://grafana.com/blog/2020/09/28/new-in-grafana-7.2-__rate_interval-for-prometheus-rate-queries-that-just-work/

Signed-off-by: Chris McDonnell <[email protected]>

Author: ChrisMcD1

internal/config/analytics.go

@@ -60,9 +60,10 @@ func (c *ClickhouseConfig) Options() (*clickhouse.Options, error) {
 
 // PrometheusConfig defines the connection details for connecting Flipt to Prometheus.
 type PrometheusConfig struct {
-	Enabled bool              `json:"enabled,omitempty" mapstructure:"enabled" yaml:"enabled,omitempty"`
-	URL     string            `json:"-" mapstructure:"url" yaml:"-"`
-	Headers map[string]string `json:"-" mapstructure:"headers" yaml:"-"`
+	Enabled               bool              `json:"enabled,omitempty" mapstructure:"enabled" yaml:"enabled,omitempty"`
+	ScrapeIntervalSeconds int               `json:"scrapeIntervalSeconds,omitempty" mapstructure:"scrapeIntervalSeconds" yaml:"scrapeIntervalSeconds,omitempty"`
+	URL                   string            `json:"-" mapstructure:"url" yaml:"-"`
+	Headers               map[string]string `json:"-" mapstructure:"headers" yaml:"-"`
 }
 
 //nolint:unparam
@@ -74,8 +75,9 @@ func (a *AnalyticsConfig) setDefaults(v *viper.Viper) error {
 				"url":     "",
 			},
 			"prometheus": map[string]any{
-				"enabled": "false",
-				"url":     "",
+				"enabled":               "false",
+				"scrapeIntervalSeconds": "15",
+				"url":                   "",
 			},
 		},
 		"buffer": map[string]any{

internal/server/analytics/prometheus/client.go

@@ -21,8 +21,9 @@ type prometheusClient interface {
 }
 
 type client struct {
-	promClient prometheusClient
-	logger     *zap.Logger
+	promClient     prometheusClient
+	minStepMinutes int
+	logger         *zap.Logger
 }
 
 func New(logger *zap.Logger, cfg *config.Config) (*client, error) {
@@ -39,15 +40,21 @@ func New(logger *zap.Logger, cfg *config.Config) (*client, error) {
 		return nil, err
 	}
 	promClient := promapi.NewAPI(apiClient)
-	return &client{promClient: promClient, logger: logger}, nil
+	return &client{
+		promClient:     promClient,
+		logger:         logger,
+		minStepMinutes: cfg.Analytics.Storage.Prometheus.ScrapeIntervalSeconds * 4 / 60,
+	}, nil
 }
 
 func (c *client) GetFlagEvaluationsCount(ctx context.Context, req *panalytics.FlagEvaluationsCountRequest) ([]string, []float32, error) {
+	stepMinutes := max(req.StepMinutes, c.minStepMinutes)
 	query := fmt.Sprintf(
-		`sum(increase(flipt_evaluations_requests_total{namespace="%s", flag="%s"}[%dm])) or vector(0)`,
+		`sum(increase(flipt_evaluations_requests_total{namespace="%s", flag="%s"}[%dm]) / %d) or vector(0)`,
 		req.NamespaceKey,
 		req.FlagKey,
-		req.StepMinutes,
+		stepMinutes,
+		stepMinutes,
 	)
 	r := promapi.Range{
 		Start: req.From.UTC(),