Merge #3061

3061: Name spawned threads r=irevoire a=dureuill

# Pull Request

## Related issue
None, this is to improve debuggability

## What does this PR do?
- This PR replaces the raw `thread::spawn(...)` calls by `thread::Builder::new().name(...).spawn(...).unwrap()` calls so that we can give meaningful names to threads. 
- This PR also setup the `rayon` thread pool to give a name to its threads.
- This improves debuggability, as the thread names are reported by debuggers:

<img width="411" alt="Capture d’écran 2022-11-16 à 10 26 27" src="https://user-images.githubusercontent.com/41078892/202141870-a88663aa-d2f8-494f-b4da-709fdbd072ba.png">

(screen showing vscode's debugger and its main/scheduler/indexing threads)

## PR checklist
Please check if your PR fulfills the following requirements:
- [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
- [x] Have you read the contributing guidelines?
- [x] Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!


Co-authored-by: Louis Dureuil <louis@meilisearch.com>

meilisearch-http/src/lib.rs

@@ -204,12 +204,15 @@ pub fn setup_meilisearch(opt: &Opt) -> anyhow::Result<(Arc<IndexScheduler>, Auth
     if opt.schedule_snapshot {
         let snapshot_delay = Duration::from_secs(opt.snapshot_interval_sec);
         let index_scheduler = index_scheduler.clone();
-        thread::spawn(move || loop {
-            thread::sleep(snapshot_delay);
-            if let Err(e) = index_scheduler.register(KindWithContent::SnapshotCreation) {
-                error!("Error while registering snapshot: {}", e);
-            }
-        });
+        thread::Builder::new()
+            .name(String::from("register-snapshot-tasks"))
+            .spawn(move || loop {
+                thread::sleep(snapshot_delay);
+                if let Err(e) = index_scheduler.register(KindWithContent::SnapshotCreation) {
+                    error!("Error while registering snapshot: {}", e);
+                }
+            })
+            .unwrap();
     }
 
     Ok((index_scheduler, auth_controller))

meilisearch-http/src/option.rs

@@ -501,8 +501,10 @@ impl TryFrom<&IndexerOpts> for IndexerConfig {
     type Error = anyhow::Error;
 
     fn try_from(other: &IndexerOpts) -> Result<Self, Self::Error> {
-        let thread_pool =
-            rayon::ThreadPoolBuilder::new().num_threads(*other.max_indexing_threads).build()?;
+        let thread_pool = rayon::ThreadPoolBuilder::new()
+            .thread_name(|index| format!("indexing-thread:{index}"))
+            .num_threads(*other.max_indexing_threads)
+            .build()?;
 
         Ok(Self {
             log_every_n: Some(other.log_every_n),