Expose nats.Context option for nats.KeyWatcher (#904)

* Expose KeyWatcher context to facilitate KeyValue interface extension

Signed-off-by: Matthew DeVenny <matt@boxboat.com>

* Expose KeyWatcher context to facilitate KeyValue interface extension

Signed-off-by: Matthew DeVenny <matt@boxboat.com>

* Rename GetContext to Context

Signed-off-by: Matthew DeVenny <matt@boxboat.com>

kv.go

@@ -94,6 +94,8 @@ type KeyValueStatus interface {
 
 // KeyWatcher is what is returned when doing a watch.
 type KeyWatcher interface {
+	// Context returns watcher context optionally provided by nats.Context option.
+	Context() context.Context
 	// Updates returns a channel to read any updates to entries.
 	Updates() <-chan KeyValueEntry
 	// Stop will stop this watcher.
@@ -743,6 +745,15 @@ type watcher struct {
 	initDone    bool
 	initPending uint64
 	received    uint64
+	ctx         context.Context
+}
+
+// Context returns the context for the watcher if set.
+func (w *watcher) Context() context.Context {
+	if w == nil {
+		return nil
+	}
+	return w.ctx
 }
 
 // Updates returns the interior channel.
@@ -785,7 +796,7 @@ func (kv *kvs) Watch(keys string, opts ...WatchOpt) (KeyWatcher, error) {
 	keys = b.String()
 
 	// We will block below on placing items on the chan. That is by design.
-	w := &watcher{updates: make(chan KeyValueEntry, 256)}
+	w := &watcher{updates: make(chan KeyValueEntry, 256), ctx: o.ctx}
 
 	update := func(m *Msg) {
 		tokens, err := getMetadataFields(m.Reply)