Merge #3851

3851: Expose lastUpdate and isIndexing in /stats endpoint r=dureuill a=gentcys

# Pull Request

## Related issue
Fixes #3843

## What does this PR do?
- expose lastUpdate in `/stats` endpoint
- expose isIndex in `stats` endpoint
- add a method `is_task_processing` in index-scheduler/src/lib.rs.

## 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: Cong Chen <cong.chen@ocrlabs.com>
Co-authored-by: ManyTheFish <many@meilisearch.com>
Co-authored-by: Louis Dureuil <louis@meilisearch.com>

meilisearch/src/metrics.rs

@@ -50,4 +50,10 @@ lazy_static! {
         &["kind", "value"]
     )
     .expect("Can't create a metric");
+    pub static ref MEILISEARCH_LAST_UPDATE: IntGauge =
+        register_int_gauge!(opts!("meilisearch_last_update", "Meilisearch Last Update"))
+            .expect("Can't create a metric");
+    pub static ref MEILISEARCH_IS_INDEXING: IntGauge =
+        register_int_gauge!(opts!("meilisearch_is_indexing", "Meilisearch Is Indexing"))
+            .expect("Can't create a metric");
 }