Add WithDialTLSContextFunc option

[LeviHarrison]

This PR adds needed functionality to implement prometheus/prometheus#4827, in the form of the WithDialTLSContextFunc option and __server_name__ label.

(slightly incorrect) Example implementation:

fn := func(ctx context.Context, network, addr string) (net.Conn, error) {
	tls := cfg.ScrapeConfig.HTTPClientConfig.TLSConfig
	tls.ServerName = ctx.Value(model.ServerNameLabel)

	dialer := &tls.Dialer{
		Config: tls,
	}
	return dialer.DialContext(ctx, network, addr)
}

NewClientFromConfig(cfg, "", WithDialTLSContextFunc(fn))

[LeviHarrison]

Unfortunately, the DialTLSContext options was only introduced in 1.14, so this patch doesn't support 1.13.

[LeviHarrison]

cc @roidelapluie

[LeviHarrison]

Do we need to do the same with DialTLSContextFunc as we do with DialContextFunc?

common/config/http_config.go

Lines 369 to 378 in d9fc5f4

	if opts.dialContextFunc != nil {
	dialContext = conntrack.NewDialContextFunc(
	conntrack.DialWithDialContextFunc((func(context.Context, string, string) (net.Conn, error))(opts.dialContextFunc)),
	conntrack.DialWithTracing(),
	conntrack.DialWithName(name))
	} else {
	dialContext = conntrack.NewDialContextFunc(
	conntrack.DialWithTracing(),
	conntrack.DialWithName(name))
	}

[roidelapluie]

No.

[roidelapluie]

To remove go 1.13, we would need to see with client_golang.

Otherwise, I'd suggest that you go get this commit in your fork so we can assess of the feature would work.

[LeviHarrison]

Build tags with version constraints could also be an option.

[roidelapluie]

Can you please rebase this so we can continue working on this?

[LeviHarrison]

Sure, but if I remember correctly what was holding this up was the usage of this new function: prometheus/prometheus#9482 (comment).