Remove inlined upstream code (prometheus#1539)

Avoid having to maintain inlined code by using Go modules for
`github.com/kylelemons/godebug`.

Signed-off-by: SuperQ <superq@gmail.com>
Signed-off-by: Steven Swartz <sswartz@ripple.com>

NOTICE

@@ -16,8 +16,3 @@ Go support for Protocol Buffers - Google's data interchange format
 http://github.com/golang/protobuf/
 Copyright 2010 The Go Authors
 See source code for license details.
-
-diff - a pretty-printed complete of a Go data structure
-https://github.com/kylelemons/godebug
-Copyright 2013 Google Inc.  All rights reserved.
-See source code for license details.

go.mod

@@ -8,6 +8,7 @@ require (
 	github.com/google/go-cmp v0.6.0
 	github.com/json-iterator/go v1.1.12
 	github.com/klauspost/compress v1.17.8
+	github.com/kylelemons/godebug v1.1.0
 	github.com/prometheus/client_model v0.6.1
 	github.com/prometheus/common v0.53.0
 	github.com/prometheus/procfs v0.15.1

go.sum

@@ -24,6 +24,8 @@ github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
 github.com/kr/pretty v0.3.1/go.mod h1:hoEshYVHaxMs3cyo3Yncou5ZscifuDolrwPKZanG3xk=
 github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
 github.com/kr/text v0.2.0/go.mod h1:eLer722TekiGuMkidMxC/pM04lWEeraHUUmBw8l2grE=
+github.com/kylelemons/godebug v1.1.0 h1:RPNrshWIDI6G2gRW9EHilWtl7Z6Sb1BR0xunSBf0SNc=
+github.com/kylelemons/godebug v1.1.0/go.mod h1:9/0rRGxNHcop5bhtWyNeEfOS8JIWk580+fNqagV/RAw=
 github.com/modern-go/concurrent v0.0.0-20180228061459-e0a39a4cb421/go.mod h1:6dJC0mAP4ikYIbvyc7fijjWJddQyLn8Ig3JB5CqoB9Q=
 github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd h1:TRLaZ9cD/w8PVh93nsPXa1VrQ6jlwL5oN8l14QlcNfg=
 github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd/go.mod h1:6dJC0mAP4ikYIbvyc7fijjWJddQyLn8Ig3JB5CqoB9Q=