Add metric for server side TLS handshake failures.

[mikkokar]

The problem

There is no clear metric for failed TLS handshake attempts. At the moment SSL handshake failures go under styx.exception.io_netty_handler_codec_DecoderException metric which is a catch-all for all sorts of decoder problems, and also not specific enough for the TLS handshake issues.

As a devops engineer, I want to know how many failed TLS handshake attempts there has been, so that I can get a clear indication on the configuration problems, and to take a remedial action sooner.

Acceptance criteria

• Add a metric, perhaps in connections scope, such as connections.tls.handshake.failures. Increment this metric on every failed TLS handshake.

• Add the new metric in the styx user manual: https://github.com/HotelsDotCom/styx/blob/master/docs/user-guide/metrics.md

• Add the new metric in the styx user manual under "Troubleshooting TLS connectivity issues". https://github.com/HotelsDotCom/styx/blob/master/docs/user-guide/configure-tls.md

[mikkokar]

There is a it("Logs an SSL handshake exception") test in TlsErrorSpec.scala to give you some ideas @VivianLopes .

[kvosper]

Closing all issues over 3 years old. New issues can be created if problems are still occurring.