Print error on Sampler Query failure (#328)

Signed-off-by: Goutham Veeramachaneni <[email protected]>

sampler.go

@@ -511,6 +511,7 @@ func (s *RemotelyControlledSampler) updateSampler() {
 	res, err := s.manager.GetSamplingStrategy(s.serviceName)
 	if err != nil {
 		s.metrics.SamplerQueryFailure.Inc(1)
+		s.logger.Infof("Unable to query sampling strategy: %v", err)
 		return
 	}
 	s.Lock()

sampler_test.go

@@ -18,6 +18,7 @@ import (
 	"errors"
 	"fmt"
 	"runtime"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -649,6 +650,18 @@ func getSamplingStrategyResponse(strategyType sampling.SamplingStrategyType, val
 	return nil
 }
 
+func TestRemotelyControlledSampler_printErrorForBrokenUpstream(t *testing.T) {
+	logger := &log.BytesBufferLogger{}
+	sampler := NewRemotelyControlledSampler(
+		"client app",
+		SamplerOptions.Logger(logger),
+	)
+	sampler.Close() // stop timer-based updates, we want to call them manually
+	sampler.updateSampler()
+
+	assert.True(t, strings.Contains(logger.String(), "Unable to query sampling strategy:"))
+}
+
 func TestAdaptiveSampler_lockRaceCondition(t *testing.T) {
 	agent, remoteSampler, _ := initAgent(t)
 	defer agent.Close()