Add snmp input option to strip non fixed length index suffixes (#4025)

plugins/inputs/snmp/README.md

@@ -141,6 +141,9 @@ OID to get. May be a numeric or textual OID.
 * `oid_index_suffix`:
 The OID sub-identifier to strip off so that the index can be matched against other fields in the table.
 
+* `oid_index_length`:
+Specifies the length of the index after the supplied table OID (in OID path segments). Truncates the index after this point to remove non-fixed value or length index suffixes.
+
 * `name`:
 Output field/tag name.
 If not specified, it defaults to the value of `oid`. If `oid` is numeric, an attempt to translate the numeric OID into a texual OID will be made.

plugins/inputs/snmp/snmp.go

@@ -237,6 +237,8 @@ type Field struct {
 	Oid string
 	// OidIndexSuffix is the trailing sub-identifier on a table record OID that will be stripped off to get the record's index.
 	OidIndexSuffix string
+	// OidIndexLength specifies the length of the index in OID path segments. It can be used to remove sub-identifiers that vary in content or length.
+	OidIndexLength int
 	// IsTag controls whether this OID is output as a tag or a value.
 	IsTag bool
 	// Conversion controls any type conversion that is done on the value.
@@ -462,6 +464,18 @@ func (t Table) Build(gs snmpConnection, walk bool) (*RTable, error) {
 					}
 					idx = idx[:len(idx)-len(f.OidIndexSuffix)]
 				}
+				if f.OidIndexLength != 0 {
+					i := f.OidIndexLength + 1 // leading separator
+					idx = strings.Map(func(r rune) rune {
+						if r == '.' {
+							i -= 1
+						}
+						if i < 1 {
+							return -1
+						}
+						return r
+					}, idx)
+				}
 
 				fv, err := fieldConvert(f.Conversion, ent.Value)
 				if err != nil {

plugins/inputs/snmp/snmp_test.go

@@ -453,6 +453,11 @@ func TestTableBuild_walk(t *testing.T) {
 				Oid:            ".1.0.0.2.1.5",
 				OidIndexSuffix: ".9.9",
 			},
+			{
+				Name:           "myfield5",
+				Oid:            ".1.0.0.2.1.5",
+				OidIndexLength: 1,
+			},
 		},
 	}
 
@@ -469,6 +474,7 @@ func TestTableBuild_walk(t *testing.T) {
 			"myfield2": 1,
 			"myfield3": float64(0.123),
 			"myfield4": 11,
+			"myfield5": 11,
 		},
 	}
 	rtr2 := RTableRow{
@@ -480,6 +486,7 @@ func TestTableBuild_walk(t *testing.T) {
 			"myfield2": 2,
 			"myfield3": float64(0.456),
 			"myfield4": 22,
+			"myfield5": 22,
 		},
 	}
 	rtr3 := RTableRow{