add metric to record tls ciphersuite negotiated during handshake

[sding3]

Record name of ciphersuite negotiated during handshake in new probe_tls_cipher_info metric, useful for evaluating ciphers in use by monitored endpoints.

Example:

$ curl http://localhost:9115/probe?target=https://google.com&module=http_2xx

<...abbreviated...>

# HELP probe_tls_cipher_info Returns the TLS cipher negotiated during handshake or 0x000 when unknown
# TYPE probe_tls_cipher_info gauge
probe_tls_cipher_info{cipher="TLS_AES_128_GCM_SHA256"} 1
# HELP probe_tls_version_info Returns the TLS version used or NaN when unknown
# TYPE probe_tls_version_info gauge
probe_tls_version_info{version="TLS 1.3"} 1

[mem]

Looks good to me, it's inline with other information that's exported.

Reading thru the tls code I'm not 100% sure when 0x0000 will be used. Unless I'm missing something, the code cannot negotiate a suite that it doesn't know about, so it seems to me that 0 actually means "negotiation failed", is that correct?

[sding3]

@mem , yes that's right, and in that case we wouldn't produce this new metric and some of the other TLS related metrics due to not being able to enter into the if resp.TLS != nil { } branch on line 644 in prober/http.go.

I've cleaned up the metric help text in b63e5ae to no longer mention 0x0000.

[mem]

@mem , yes that's right, and in that case we wouldn't produce this new metric and some of the other TLS related metrics due to not being able to enter into the if resp.TLS != nil { } branch on line 644 in prober/http.go.

I've cleaned up the metric help text in b63e5ae to no longer mention 0x0000.

Thanks, looks good.

You will need to sign the DCO in order to get this merged.

[sding3]

Oops, the DCO in the commit message should be fixed now.

[sding3]

@mem - friendly ping.

[mem]

Oops, sorry, I missed this notification.