breaking change: modify API for working with response metadata (#213)

* implement functions to append/get metadata and set/get status code

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

* сhanged work with HTTP headers to use direct array assignment instead of for-range

* fix linters

* fix meter handler

* fix uninitialized response metadata for incoming context

* removed a useless test

* metrics handler has been fixed to work with compressed data

response.go

@@ -0,0 +1,29 @@
+package http
+
+import (
+	"context"
+	"net/http"
+)
+
+type (
+	rspStatusCodeKey struct{}
+	rspStatusCodeVal struct {
+		code int
+	}
+)
+
+// SetResponseStatusCode sets the status code in the context.
+func SetResponseStatusCode(ctx context.Context, code int) {
+	if rsp, ok := ctx.Value(rspStatusCodeKey{}).(*rspStatusCodeVal); ok {
+		rsp.code = code
+	}
+}
+
+// GetResponseStatusCode retrieves the response status code from the context.
+func GetResponseStatusCode(ctx context.Context) int {
+	code := http.StatusOK
+	if rsp, ok := ctx.Value(rspStatusCodeKey{}).(*rspStatusCodeVal); ok {
+		code = rsp.code
+	}
+	return code
+}