Create CONFIG-EXAMPLES.md with a switch interface example

Added a standard example for collecting interface metrics from switches or routers and tagging them properly.

closes #1666

plugins/inputs/snmp/CONFIG-EXAMPLES.md

@@ -0,0 +1,65 @@
+Here are a few configuration examples for different use cases.
+
+### Switch/router interface metrics
+
+This setup will collect data on all interfaces from three different tables, `IF-MIB::ifTable`, `IF-MIB::ifXTable` and `EtherLike-MIB::dot3StatsTable`. It will also add the name from `IF-MIB::ifDescr` and use that as a tag. Depending on your needs and preferences you can easily use `IF-MIB::ifName` or `IF-MIB::ifAlias` instead or in addition. The values of these are typically:
+
+    IF-MIB::ifName = Gi0/0/0
+    IF-MIB::ifDescr = GigabitEthernet0/0/0
+    IF-MIB::ifAlias = ### LAN ###
+
+This configuration also collects the hostname from the device (`RFC1213-MIB::sysName.0`) and adds as a tag. So each metric will both have the configured host/IP as `agent_host` as well as the device self-reported hostname as `hostname` and the name of the host that has collected these metrics as `host`.
+
+Here is the configuration that you add to your `telegraf.conf`:
+
+```
+[[inputs.snmp]]
+  agents = [ "host.example.com" ]
+  version = 2
+  community = "public"
+
+  [[inputs.snmp.field]]
+    name = "hostname"
+    oid = "RFC1213-MIB::sysName.0"
+    is_tag = true
+
+  [[inputs.snmp.field]]
+    name = "uptime"
+    oid = "DISMAN-EXPRESSION-MIB::sysUpTimeInstance"
+
+  # IF-MIB::ifTable contains counters on input and output traffic as well as errors and discards.
+  [[inputs.snmp.table]]
+    name = "interface"
+    inherit_tags = [ "hostname" ]
+    oid = "IF-MIB::ifTable"
+
+    # Interface tag - used to identify interface in metrics database
+    [[inputs.snmp.table.field]]
+      name = "ifDescr"
+      oid = "IF-MIB::ifDescr"
+      is_tag = true
+
+  # IF-MIB::ifXTable contains newer High Capacity (HC) counters that do not overflow as fast for a few of the ifTable counters
+  [[inputs.snmp.table]]
+    name = "interface"
+    inherit_tags = [ "hostname" ]
+    oid = "IF-MIB::ifXTable"
+
+    # Interface tag - used to identify interface in metrics database
+    [[inputs.snmp.table.field]]
+      name = "ifDescr"
+      oid = "IF-MIB::ifDescr"
+      is_tag = true
+
+  # EtherLike-MIB::dot3StatsTable contains detailed ethernet-level information about what kind of errors have been logged on an interface (such as FCS error, frame too long, etc)
+  [[inputs.snmp.table]]
+    name = "interface"
+    inherit_tags = [ "hostname" ]
+    oid = "EtherLike-MIB::dot3StatsTable"
+
+    # Interface tag - used to identify interface in metrics database
+    [[inputs.snmp.table.field]]
+      name = "ifDescr"
+      oid = "IF-MIB::ifDescr"
+      is_tag = true
+```

plugins/inputs/snmp/README.md

@@ -4,6 +4,8 @@ The SNMP input plugin gathers metrics from SNMP agents.
 
 ## Configuration:
 
+See additional SNMP plugin configuration examples [here](./CONFIG-EXAMPLES.md).
+
 ### Example:
 
 SNMP data: