[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>

metadata.go

@@ -0,0 +1,47 @@
+package grpc
+
+import (
+	"context"
+
+	"go.unistack.org/micro/v4/metadata"
+)
+
+type (
+	rspMetadataKey struct{}
+	rspMetadataVal struct {
+		m metadata.Metadata
+	}
+)
+
+// AppendResponseMetadata adds metadata entries to metadata.Metadata stored in the context.
+// It expects the context to contain a *rspMetadataVal value under the rspMetadataKey{} key.
+// If the value is missing or invalid, the function does nothing.
+//
+// Note: this function is not thread-safe. Synchronization is required if used from multiple goroutines.
+func AppendResponseMetadata(ctx context.Context, md metadata.Metadata) {
+	if md == nil {
+		return
+	}
+
+	val, ok := ctx.Value(rspMetadataKey{}).(*rspMetadataVal)
+	if !ok || val == nil || val.m == nil {
+		return
+	}
+
+	for key, values := range md {
+		val.m.Append(key, values...)
+	}
+}
+
+// getResponseMetadata retrieves the metadata.Metadata stored in the context.
+//
+// Note: this function is not thread-safe. Synchronization is required if used from multiple goroutines.
+// If you plan to modify the returned metadata, make a full copy to avoid affecting shared state.
+func getResponseMetadata(ctx context.Context) metadata.Metadata {
+	val, ok := ctx.Value(rspMetadataKey{}).(*rspMetadataVal)
+	if !ok || val == nil || val.m == nil {
+		return nil
+	}
+
+	return val.m
+}