hostmetrics: don't warn on missing metrics (#177)

Don't warn if system.cpu.logical.count is missing,
as it's an optional metric. The warning is particularly
unhelpful in gateway deployments, where the central
operations team has no control over what is being sent
to the gateway.

Author: axw

remappers/hostmetrics/cpu.go

@@ -18,8 +18,6 @@
 package hostmetrics
 
 import (
-	"errors"
-
 	"go.opentelemetry.io/collector/pdata/pcommon"
 	"go.opentelemetry.io/collector/pdata/pmetric"
 
@@ -146,7 +144,10 @@ func remapCPUMetrics(
 	)
 
 	if numCores == 0 {
-		return errors.New("system.cpu.logical.count metric is missing in the hostmetrics")
+		// system.cpu.logical.count is optional, and if it's not
+		// present we can't calculate the normalized values.
+		// This is not an error, so we just return early.
+		return nil
 	}
 
 	totalNorm := totalPercent / float64(numCores)

remappers/hostmetrics/hostmetrics_test.go

@@ -29,7 +29,10 @@ import (
 	"github.com/stretchr/testify/assert"
 	"go.opentelemetry.io/collector/pdata/pcommon"
 	"go.opentelemetry.io/collector/pdata/pmetric"
+	"go.uber.org/zap"
+	"go.uber.org/zap/zapcore"
 	"go.uber.org/zap/zaptest"
+	"go.uber.org/zap/zaptest/observer"
 )
 
 var (
@@ -348,10 +351,16 @@ func doTestRemap(t *testing.T, id string, remapOpts ...Option) {
 			resource := pcommon.NewResource()
 			resource.Attributes().FromRaw(tc.resourceAttrs)
 
+			core, observedLogs := observer.New(zapcore.WarnLevel)
+			logger := zap.New(core)
+
 			actual := pmetric.NewMetricSlice()
-			r := NewRemapper(zaptest.NewLogger(t), remapOpts...)
+			r := NewRemapper(logger, remapOpts...)
 			r.Remap(sm, actual, resource)
 			assert.Empty(t, cmp.Diff(tc.expected, testutils.MetricSliceToTestMetric(t, actual), cmpopts.EquateApprox(0, 0.001)))
+
+			// There should be no warning or error logs.
+			assert.Zero(t, observedLogs.Len())
 		})
 	}
 }