`prober.DERP` was created in #5988 based on derpprobe. Having used it
instead of derpprobe for a few months, I think we have enough confidence
that it works and can now migrate derpprobe to use the prober framework
and get rid of code duplication.
A few notable changes in behaviour:
- results of STUN probes over IPv4 and IPv6 are now reported separately;
- TLS probing now includes OCSP verification;
- probe names in the output have changed;
- ability to send Slack notification from the prober has been removed.
Instead, the prober now exports metrics in Expvar (/debug/vars) and
Prometheus (/debug/varz) formats.
Fixes https://github.com/tailscale/corp/issues/8497
Signed-off-by: Anton Tolchanov <anton@tailscale.com>
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