smart diff to testutil.GatherAndCompare (#998)

* added smart diff to testutil.GatherAndCompare

Signed-off-by: Sourik Ghosh <[email protected]>

* v2 testuitls added for better diff

Signed-off-by: Sourik Ghosh <[email protected]>

* replaced CollectAndCompare to CollectAndCompareV2 in testutil_test

Signed-off-by: Sourik Ghosh <[email protected]>

* renamed methods from v2 to withT

Signed-off-by: Sourik Ghosh <[email protected]>

* replaced testify with custom diff func

Signed-off-by: Sourik Ghosh <[email protected]>

* difflib GetUnifiedDiffString added with test

Signed-off-by: Sourik Ghosh <[email protected]>

* license meta data added to file

Signed-off-by: Sourik Ghosh <[email protected]>

* moved difflib to internal

Signed-off-by: Sourik Ghosh <[email protected]>

Author: sourikghosh

go.mod

@@ -3,6 +3,7 @@ module github.com/prometheus/client_golang
 require (
 	github.com/beorn7/perks v1.0.1
 	github.com/cespare/xxhash/v2 v2.1.2
+	github.com/davecgh/go-spew v1.1.1
 	github.com/golang/protobuf v1.5.2
 	github.com/jpillora/backoff v1.0.0 // indirect
 	github.com/json-iterator/go v1.1.12