ifName `agent` default value

Feature Request

Opening a feature request kicks off a discussion. As discussed in https://github.com/influxdata/telegraf/pull/7763#issuecomment-676801651

Proposal:

Let the default value of these be the same:

• agent for processors.ifname
• agent_host for inputs.snmp
• agent_address for inputs.sflow

Current behavior:

The default value (agent) mostly never useful.

Desired behavior:

The default value should be a logical one that makes sense or most users would not need to change.

• #14111
• #14112
• #14117
• #14899