Set HTTP hostname based on TLS server name

[fpetkovski]

The net/http library uses the Host field from the Request object in order to determine the value of the Host header [1]. In order for the Prometheus client to support SNI, it needs to set this field to the value provided in the TLS server name.

[1] https://github.com/golang/go/issues/29865

Signed-off-by: fpetkovski [email protected]

Fixes https://github.com/prometheus/prometheus/issues/9403

[roidelapluie]

Hello,

Thank you for your contribution. I am surprised that SNI does not work out of the box.

I am not willing to have this repository depend on client_golang. We have an exception for an interface in version, and the tests in sigv4, but that's it.

[fpetkovski]

Thanks for the feedback. Could you point me to where the dependency to client_golang is introduced in this PR? Do you mean because we are referencing the TLS config?

[roidelapluie]

The initial commit introduced a dependency. Then it was force pushed.

I can not merge this as is, I need to check that 1. SNI works even with our connection pooling and 2. This does not break blackbox exporter, which has similar logic.

[fpetkovski]

Ah yes, that seems to be correct: https://github.com/prometheus/common/commit/038724fffa024b190650f1cf95adaf7fd57eaa96#diff-cdb28b391884a82ef481177749e35ba9c3c54bda95d93ffe3d5f3ba1c67c2c5eR35

[roidelapluie]

after thinking twice, the TLS servername should come from the header, not the other way around.

[fpetkovski]

What do you mean by this? Isn't the TLS server name a user provided config?