(#5024: Add option to store cpu as a tag in interrupts input) breaks compatibility

[bofh16]

Relevant telegraf.conf:

[[inputs.interrupts]]
        cpu_as_tag = false

System info:

Linux 3.10.0-957.12.1.el7.x86_64 #1 SMP Mon Apr 29 14:59:59 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
CentOS Linux release 7.6.1810 (Core)

telegraf --version
Telegraf 1.10.3 (git: HEAD 294bb666)

Steps to reproduce:

> show field keys;
name: soft_interrupts
fieldKey fieldType
-------- ---------
CPU0 integer
CPU1 integer
CPU2 integer
CPU3 integer
count integer
cpu0 integer
cpu1 integer
cpu2 integer
cpu3 integer
total integer

Expected behavior:

Fieldkeys format is expected to be preserved between versions, as well as any other filed, previously used. Not doing so, leaves the data, collected so far, "behind".

Actual behavior:

Data collection is imported in the lower letters keys cpu[0-9] and count. The behavior has changed since v1.10.

Additional info:

[bofh16]

This might be related.
A new database has been created for a new telegraf client, which has been configured explicitly with the new setting from the beginning.

[[inputs.interrupts]]
cpu_as_tag = true

The result:

> select * from soft_interrupts limit 10;
name: soft_interrupts
time count cpu host irq
---- ----- --- ---- ---
1557827410000000000 15486601 cpu0 webstore BLOCK

> show field keys;
Only this!
name: soft_interrupts
fieldKey fieldType
-------- ---------
count integer

After changing to false and back to true:

> select * from soft_interrupts limit 10;
name: soft_interrupts
time count cpu cpu0 cpu1 cpu2 cpu3 cpu4 cpu5 cpu6 cpu7 host irq total
---- ----- --- ---- ---- ---- ---- ---- ---- ---- ---- ---- --- -----
1557827410000000000 15486601 cpu0 webstore BLOCK

> show field keys;
name: soft_interrupts
fieldKey fieldType
-------- ---------
count integer
cpu0 integer
cpu1 integer
cpu2 integer
cpu3 integer
cpu4 integer
cpu5 integer
cpu6 integer
cpu7 integer
total integer