Add tags to checkpoints in kopia (#2153)

## Description

Tags allow searching for previous snapshots (complete or otherwise). This in turn provides the ability to use kopia-assisted incrementals in more situations as partially completed backups will be reused for their cached data.

Streaming files that have not been completely uploaded are not added to a checkpoint so there is no possibility of accidentally triggering kopia-assisted incrementals thus causing us to treat a partially uploaded file in a checkpoint as a "cached" version of the complete file

## Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No 

## Type of change

- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Test
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

## Issue(s)

* closes #1674 

## Test Plan

- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/internal/kopia/snapshot_manager.go

@@ -24,6 +24,9 @@ const (
 	// (permalinks)
 	// [1] https://github.com/kopia/kopia/blob/05e729a7858a6e86cb48ba29fb53cb6045efce2b/cli/command_snapshot_create.go#L169
 	userTagPrefix = "tag:"
+
+	// Tag key applied to checkpoints (but not completed snapshots) in kopia.
+	checkpointTagKey = "checkpoint"
 )
 
 type Reason struct {

src/internal/kopia/wrapper.go

@@ -192,6 +192,24 @@ func (w Wrapper) makeSnapshotWithRoot(
 		snapIDs,
 	)
 
+	checkpointTagK, checkpointTagV := makeTagKV(checkpointTagKey)
+
+	tags := map[string]string{}
+	checkpointTags := map[string]string{
+		checkpointTagK: checkpointTagV,
+	}
+
+	for k, v := range addlTags {
+		mk, mv := makeTagKV(k)
+
+		if len(v) == 0 {
+			v = mv
+		}
+
+		tags[mk] = v
+		checkpointTags[mk] = v
+	}
+
 	err := repo.WriteSession(
 		ctx,
 		w.c,
@@ -228,6 +246,7 @@ func (w Wrapper) makeSnapshotWithRoot(
 			u := snapshotfs.NewUploader(rw)
 			progress.UploadProgress = u.Progress
 			u.Progress = progress
+			u.CheckpointLabels = checkpointTags
 
 			man, err = u.Upload(innerCtx, root, policyTree, si, prevSnaps...)
 			if err != nil {
@@ -236,17 +255,7 @@ func (w Wrapper) makeSnapshotWithRoot(
 				return err
 			}
 
-			man.Tags = map[string]string{}
-
-			for k, v := range addlTags {
-				mk, mv := makeTagKV(k)
-
-				if len(v) == 0 {
-					v = mv
-				}
-
-				man.Tags[mk] = v
-			}
+			man.Tags = tags
 
 			if _, err := snapshot.SaveSnapshot(innerCtx, rw, man); err != nil {
 				err = errors.Wrap(err, "saving snapshot")