Fix snmp overriding of auto-configured table fields (#4208)

Whenever the snmp plugin was configured with a table with automatic field
discovery, if one of those fields was explicitly overridden in the config and
the value of is_tag was changed, the field would be duplicated, once as a tag
& once as a field.

This change fixes the behavior so that if a field is explicitly configured,
automatic table field discovery doesn't touch it.

plugins/inputs/snmp/snmp.go

@@ -218,10 +218,20 @@ func (t *Table) initBuild() error {
 	if err != nil {
 		return err
 	}
+
 	if t.Name == "" {
 		t.Name = oidText
 	}
-	t.Fields = append(t.Fields, fields...)
+
+	knownOIDs := map[string]bool{}
+	for _, f := range t.Fields {
+		knownOIDs[f.Oid] = true
+	}
+	for _, f := range fields {
+		if !knownOIDs[f.Oid] {
+			t.Fields = append(t.Fields, f)
+		}
+	}
 
 	return nil
 }