Move goautoneg to external dependency

`goautoneg` module was imported to the repo as it was hosted in
bitbucket and required Mercurial at the time (2015).

Bitbucket repository is long gone and a GitHub fork is available under
`munnerz/goautoneg`. This fork is used by other projects like
Kubernetes.

Signed-off-by: Mikel Olasagasti Uranga <mikel@olasagasti.info>

expfmt/encode.go

@@ -21,7 +21,7 @@ import (
 	"google.golang.org/protobuf/encoding/protodelim"
 	"google.golang.org/protobuf/encoding/prototext"
 
-	"github.com/prometheus/common/internal/bitbucket.org/ww/goautoneg"
+	"github.com/munnerz/goautoneg"
 	"github.com/prometheus/common/model"
 
 	dto "github.com/prometheus/client_model/go"

go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/go-kit/log v0.2.1
 	github.com/google/go-cmp v0.6.0
 	github.com/julienschmidt/httprouter v1.3.0
+	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822
 	github.com/mwitkow/go-conntrack v0.0.0-20190716064945-2f068394615f
 	github.com/prometheus/client_model v0.6.0
 	golang.org/x/net v0.22.0

go.sum

@@ -28,6 +28,8 @@ github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
 github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
 github.com/kr/text v0.1.0 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
 github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
+github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 h1:C3w9PqII01/Oq1c1nUAm88MOHcQC9l5mIlSMApZMrHA=
+github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822/go.mod h1:+n7T8mK8HuQTcFwEeznm/DIxMOiR9yIdICNftLE1DvQ=
 github.com/mwitkow/go-conntrack v0.0.0-20190716064945-2f068394615f h1:KUppIJq7/+SVif2QVs3tOP0zanoHgBEVAwHxUSIzRqU=
 github.com/mwitkow/go-conntrack v0.0.0-20190716064945-2f068394615f/go.mod h1:qRWi+5nqEBWmkhHvq77mSJWrCKwh8bxhgT7d/eI7P4U=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=