Add log messages about metadata saved/used (#2551)

## Description

Record the number of entries in the saved metadata. This could help catch odd situations like not finding anything to backup.

## 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

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

## Issue(s)

* #2550

## Test Plan

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

src/internal/connector/exchange/service_iterators.go

@@ -50,6 +50,11 @@ func filterContainersAndFillCollections(
 		tombstones = makeTombstones(dps)
 	)
 
+	logger.Ctx(ctx).Infow(
+		"filling collections",
+		"metadata_count",
+		len(dps))
+
 	// TODO(rkeepers): this should be passed in from the caller, probably
 	// as an interface that satisfies the NewCollection requirements.
 	// But this will work for the short term.
@@ -185,6 +190,11 @@ func filterContainersAndFillCollections(
 		graph.NewMetadataEntry(graph.PreviousPathFileName, currPaths),
 	}
 
+	logger.Ctx(ctx).Infow(
+		"adding metadata collection entries",
+		"num_paths_entries", len(currPaths),
+		"num_deltas_entries", len(deltaURLs))
+
 	if len(deltaURLs) > 0 {
 		entries = append(entries, graph.NewMetadataEntry(graph.DeltaURLsFileName, deltaURLs))
 	}

src/internal/connector/onedrive/collections.go

@@ -295,6 +295,18 @@ func (c *Collections) Get(
 		prevDelta := prevDeltas[driveID]
 		oldPaths := oldPathsByDriveID[driveID]
 
+		numOldDelta := 0
+		if len(prevDelta) > 0 {
+			numOldDelta++
+		}
+
+		logger.Ctx(ctx).Infow(
+			"previous metadata for drive",
+			"num_paths_entries",
+			len(oldPaths),
+			"num_deltas_entries",
+			numOldDelta)
+
 		delta, paths, excluded, err := collectItems(
 			ctx,
 			c.itemPagerFunc(
@@ -312,6 +324,9 @@ func (c *Collections) Get(
 			return nil, nil, err
 		}
 
+		// Used for logging below.
+		numDeltas := 0
+
 		// It's alright to have an empty folders map (i.e. no folders found) but not
 		// an empty delta token. This is because when deserializing the metadata we
 		// remove entries for which there is no corresponding delta token/folder. If
@@ -319,6 +334,7 @@ func (c *Collections) Get(
 		// for collections when not actually getting delta results.
 		if len(delta.URL) > 0 {
 			deltaURLs[driveID] = delta.URL
+			numDeltas++
 		}
 
 		// Avoid the edge case where there's no paths but we do have a valid delta
@@ -329,6 +345,13 @@ func (c *Collections) Get(
 		maps.Copy(folderPaths[driveID], paths)
 
 		maps.Copy(excludedItems, excluded)
+
+		logger.Ctx(ctx).Infow(
+			"persisted metadata for drive",
+			"num_paths_entries",
+			len(paths),
+			"num_deltas_entries",
+			numDeltas)
 	}
 
 	observe.Message(ctx, observe.Safe(fmt.Sprintf("Discovered %d items to backup", c.NumItems)))