Add support for TLS DNS name allowed lists

[isodude]

Feature Request

Support for allowing only a certain list of dns names or fingerprints in client certificates

Proposal:

Current behavior:

Only filtering with CA is possible

Desired behavior:

Possible to define
tls_allowed_dns_names = [""]
tls_allowed_fingerprints = [""]

Such that only defines names or fingerprints is allowed to connect to telegraf

Use case:

Locking down access to telegraf even though the CA is shared along with other purposes.
DNS Names allow for a more dynamic approach which fingerprints allow to really pin the certificate down to exactly one.

The code is complete and I can make a PR with it.

[isodude]

Code can be viewed here: https://github.com/isodude/telegraf/tree/tls

[akrantz01]

Thanks for looking into this. Please make a PR, so your changes are easier to find.

[isodude]

Sure, I'll try to make some tests along with it.

[isodude]

Not forgetting about this, just.. redoing a lot internally.

Setting up an ACME server makes the fingerprint list kind of not needed. DNS-names however is quite handy.