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/testdata/test.mib

@@ -22,6 +22,7 @@ TestTableEntry ::=
 		server OCTET STRING,
 		connections  INTEGER,
 		latency  OCTET STRING,
+		description OCTET STRING,
 	}
 
 server OBJECT-TYPE
@@ -42,6 +43,12 @@ latency OBJECT-TYPE
 	STATUS current
 	::= { testTableEntry 3 }
 
+description OBJECT-TYPE
+	SYNTAX OCTET STRING
+	MAX-ACCESS read-only
+	STATUS current
+	::= { testTableEntry 4 }
+
 hostname OBJECT-TYPE
 	SYNTAX OCTET STRING
 	MAX-ACCESS read-only