Add source_address to HTTPClientConfig

[bobrik]

This unlocks setting source_ip_address for HTTP probes in blackbox_exporter.

[bobrik]

prometheus/blackbox_exporter#765 is how it's supposed to be used.

Not sure what to do with go1.9 and go1.10 failures.

[roidelapluie]

Can you rebase this ?

[bobrik]

Done, the conflict was in imports:

$ git diff config/http_config.go
diff --cc config/http_config.go
index 07de306,c0c4d5b..0000000
--- a/config/http_config.go
+++ b/config/http_config.go
@@@ -17,7 -17,8 +17,8 @@@ package confi

  import (
        "bytes"
+       "context"
 -      "crypto/md5"
 +      "crypto/sha256"
        "crypto/tls"
        "crypto/x509"
        "fmt"

[roidelapluie]

I think that it is not necessary to change common, that we could do everything in blackbox exporter.

[bobrik]

Here's what I came up with without changing prometheus/common: prometheus/blackbox_exporter#766. It leaks some details about use of tracing and how dialer is set up otherwise in prometheus/common.

[SuperQ]

This needs a rebase.