Collect better stats for snapshot loading (SPO)

[vladsud]

This PR

• Collects better telemetry about tree / blob structure of snapshots. In particular, we want to know.
  • number of blobs vs. number of blob leaf nodes in the tree. This difference will be influenced by attachment blobs (missing from the snapshot) and any blob deduping service does. That said, this PR stops short of deduping these cases.
  • Provide better stats on number of trees, include leaf trees. We have "leafTrees" but it was counting all trees, not just leaf nodes.
• Added "fast" parsing case for empty trees
• Added asserts in unit tests that validate our telemetry about slow parsing paths is correct.
• Deleted second version of evalBlobsAndTrees function that was not exported from package and was not used.

[msfluid-bot]

⯅ @fluid-example/bundle-size-tests: +254 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	452.49 KB	452.49 KB	■ No change
azureClient.js	545.05 KB	545.05 KB	■ No change
connectionState.js	680 Bytes	680 Bytes	■ No change
containerRuntime.js	255.4 KB	255.4 KB	■ No change
fluidFramework.js	344.58 KB	344.58 KB	■ No change
loader.js	129.36 KB	129.36 KB	■ No change
map.js	41.35 KB	41.35 KB	■ No change
matrix.js	143.61 KB	143.61 KB	■ No change
odspClient.js	513.28 KB	513.36 KB	⯅ +83 Bytes
odspDriver.js	97.29 KB	97.37 KB	⯅ +83 Bytes
odspPrefetchSnapshot.js	41.86 KB	41.94 KB	⯅ +88 Bytes
sharedString.js	161.38 KB	161.38 KB	■ No change
sharedTree.js	334.69 KB	334.69 KB	■ No change
Total Size	3.14 MB	3.14 MB	⯅ +254 Bytes

---

Baseline commit: 7829364

Generated by 🚫 dangerJS against 501c9fb

[pragya91]

Nit: suggestion to place the function next to the getTreeStats above since they are related.

Or better yet, we can move these tree stat calculating functions to a different file or odspUtils

[vladsud]

Good suggestion, but let's do it in separate PR. I prefer to separate mechanical changes (moving things around) and non-mechanical changes - easier to review / go back in time and figure out where regressions occurred.

[jatgarg]

Take a look at some small comments above and you can merge. Thanks.