[v4] breaking change: modify API for working with response metadata (#255)

* implement functions to append/get metadata

* сhanged behavior to return nil instead of empty metadata for getResponseMetadata()

* removed metadata copy when passing to gRPC headers

---------

Co-authored-by: Vasiliy Tolstov <v.tolstov@unistack.org>

go.mod

@@ -5,6 +5,7 @@ go 1.23.0
 toolchain go1.24.2
 
 require (
+	github.com/stretchr/testify v1.10.0
 	go.unistack.org/micro/v4 v4.1.8
 	golang.org/x/net v0.39.0
 	google.golang.org/grpc v1.72.0
@@ -13,8 +14,10 @@ require (
 
 require (
 	github.com/ash3in/uuidv8 v1.2.0 // indirect
+	github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/matoous/go-nanoid v1.5.1 // indirect
+	github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 // indirect
 	github.com/spf13/cast v1.7.1 // indirect
 	go.unistack.org/micro-proto/v4 v4.1.0 // indirect
 	golang.org/x/sys v0.32.0 // indirect