snmp: Don't drop lines with an empty tag

The changes in #1848 resulted in lines being dropped if they had an empty
tag. So let's count empty tags as valid to take the right decision!

plugins/inputs/snmp/snmp.go

@@ -440,6 +440,7 @@ func (s *Snmp) gatherTable(acc telegraf.Accumulator, gs snmpConnection, t Table,
 // Build retrieves all the fields specified in the table and constructs the RTable.
 func (t Table) Build(gs snmpConnection, walk bool) (*RTable, error) {
 	rows := map[string]RTableRow{}
+	emptyTags := map[string]int{}
 
 	tagCount := 0
 	for _, f := range t.Fields {
@@ -475,10 +476,8 @@ func (t Table) Build(gs snmpConnection, walk bool) (*RTable, error) {
 				if err != nil {
 					return nil, Errorf(err, "converting %q", ent.Value)
 				}
-				if fvs, ok := fv.(string); !ok || fvs != "" {
 				ifv[""] = fv
 			}
-			}
 		} else {
 			err := gs.Walk(oid, func(ent gosnmp.SnmpPDU) error {
 				if len(ent.Name) <= len(oid) || ent.Name[:len(oid)+1] != oid+"." {
@@ -498,9 +497,7 @@ func (t Table) Build(gs snmpConnection, walk bool) (*RTable, error) {
 				if err != nil {
 					return Errorf(err, "converting %q", ent.Value)
 				}
-				if fvs, ok := fv.(string); !ok || fvs != "" {
 				ifv[idx] = fv
-				}
 				return nil
 			})
 			if err != nil {
@@ -518,8 +515,10 @@ func (t Table) Build(gs snmpConnection, walk bool) (*RTable, error) {
 				rtr.Fields = map[string]interface{}{}
 				rows[i] = rtr
 			}
+			// don't add an empty string
+			if vs, ok := v.(string); !ok || vs != "" {
 				if f.IsTag {
-				if vs, ok := v.(string); ok {
+					if ok {
 						rtr.Tags[f.Name] = vs
 					} else {
 						rtr.Tags[f.Name] = fmt.Sprintf("%v", v)
@@ -527,6 +526,10 @@ func (t Table) Build(gs snmpConnection, walk bool) (*RTable, error) {
 				} else {
 					rtr.Fields[f.Name] = v
 				}
+			} else if f.IsTag {
+				// count that we had an empty tag so that we don't reject the row for wrong reasons
+				emptyTags[i]++
+			}
 		}
 	}
 
@@ -535,8 +538,8 @@ func (t Table) Build(gs snmpConnection, walk bool) (*RTable, error) {
 		Time: time.Now(), //TODO record time at start
 		Rows: make([]RTableRow, 0, len(rows)),
 	}
-	for _, r := range rows {
-		if len(r.Tags) < tagCount {
+	for i, r := range rows {
+		if len(r.Tags)+emptyTags[i] < tagCount {
 			// don't add rows which are missing tags, as without tags you can't filter
 			continue
 		}

plugins/inputs/snmp/snmp_test.go

@@ -406,9 +406,17 @@ func TestTableBuild_walk(t *testing.T) {
 			"myfield4": 22,
 		},
 	}
-	assert.Len(t, tb.Rows, 2)
+	rtr3 := RTableRow{
+		Tags: map[string]string{},
+		Fields: map[string]interface{}{
+			"myfield2": 0,
+			"myfield3": float64(0.0),
+		},
+	}
+	assert.Len(t, tb.Rows, 3)
 	assert.Contains(t, tb.Rows, rtr1)
 	assert.Contains(t, tb.Rows, rtr2)
+	assert.Contains(t, tb.Rows, rtr3)
 }
 
 func TestTableBuild_noWalk(t *testing.T) {