ProbeFunc is a function that probes something and reports whether
the probe succeeded. The provided context's deadline must be obeyed
for correct probe scheduling.
HTTP returns a Probe that healthchecks an HTTP URL.
The ProbeFunc sends a GET request for url, expects an HTTP 200
response, and verifies that want is present in the response
body. If the URL is HTTPS, the probe further checks that the TLS
certificate is good for at least the next 7 days.
TLS returns a Probe that healthchecks a TLS endpoint.
The ProbeFunc connects to hostname, does a TLS handshake, verifies
that the hostname matches the presented certificate, and that the
certificate expires in more than 7 days from the probe time.