Exclude aggregate interface 'all' thas has missing error/drop fields

[hpaluch-pil]

Hello!
When I tried stock "net_alert" examples for Telegraf I found spurious errors in my /var/log/kapacitor/kapacitor.log:

lvl=error msg="cannot perform derivative" service=kapacitor task_master=main task=net_alert_stream
node=derivative3 err="field is the wrong type" field=err_out type=<nil>

After a bit of debug it turned out that it is caused by Telegraf inserting data for virtual interface all that is
missing some fields, including these error and drop fields required by these Tasks.

So excluding interface all from results seems to fix this problem.

Please let me know what you think about this issue and proposed solution.

Required for all non-trivial PRs

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated
• Sign CLA (if not already signed)

[docmerlin]

@hpaluch-pil Hey, man, this looks great, but I can't accept it without you signing the CLA :-( Without that, our lawyers won't let us accept code from outside PRs.

[docmerlin]

@hpaluch-pil hey could you sign the CLA :-( Without that, our lawyers won't let us accept code from outside PRs.

[hpaluch-pil]

Hi @docmerlin !
I can sign CLA myself. But I don't dare to push my employer to sign CCLA, because it may cause substantial legal obligations and/or risks.