Feature Request

Some mail servers I support, cannot be monitored through telegraf's x509_cert input plugin. This happens, because these services only expose certificates after receiving a STARTTLS SMTP message.

I believe, this will also make the plugin more complete as per its current description.

Proposal:

Add support for SMTP+STARTTLS through telegraf's x509_cert input plugin.

Current behavior:

Using a source like tcp://smtp.example.com:587, will produce the following error:

[inputs.x509_cert] Error in plugin: cannot get SSL cert 'tcp://smtp.example.com:587': tls: first record does not look like a TLS handshake

Desired behavior:

Using something like smtp+starttls://smtp.example.com:587 will produce the certificates as usual for HTTPS connections and other TLS-wrapped protocols.

Use case:

The use case is to support certificate monitoring for SMTP servers that do not have a TLS-wrapped endpoint and will only support TLS with STARTTLS in the long term.