Only register grpc TLS metrics on successful handshake

[mattb18]

In the current gRPC implementation, TLS metrics are registered before the handshake is complete. In cases where the probe fails, probeSSLEarliestCertExpiryGauge is set to the default time.Time value (far in the past), resulting in any alerts using probe_ssl_earliest_cert_expiry metric firing.

In the case of the TLS handshake failing, it makes more sense for TLS metrics not to be set. This is the way the http probe is currently configured:

blackbox_exporter/prober/http.go

Line 645 in 7e25c6f

registry.MustRegister(probeSSLEarliestCertExpiryGauge, probeTLSVersion, probeTLSCipher, probeSSLLastChainExpiryTimestampSeconds, probeSSLLastInformation)

This should also solve #1120.

[mattb18]

Seems like the gRPC tests seem to be failing on all recent merge requests? I'm not able to reproduce this failure locally.

[electron0zero]

thank you for your contribution 👏🏼