feat(scanner): add scanner budgets and progress metrics (#3145)

* fix(scanner): preserve maintenance scan cadence

* feat(scanner): add scanner concurrency budget

* feat(scanner): expose scanner runtime progress

* fix(scanner): address scanner review feedback

---------

Co-authored-by: Henry Guo <marshawcoco@users.noreply.github.com>
Co-authored-by: houseme <housemecn@gmail.com>

crates/obs/src/metrics/schema/entry/metric_name.rs

@@ -278,6 +278,7 @@ pub enum MetricName {
     ScannerObjectsScanned,
     ScannerVersionsScanned,
     ScannerLastActivitySeconds,
+    ScannerActivePaths,
 
     // CPU system-related metrics
     SysCPUAvgIdle,
@@ -618,6 +619,7 @@ impl MetricName {
             Self::ScannerObjectsScanned => "objects_scanned".to_string(),
             Self::ScannerVersionsScanned => "versions_scanned".to_string(),
             Self::ScannerLastActivitySeconds => "last_activity_seconds".to_string(),
+            Self::ScannerActivePaths => "active_paths".to_string(),
 
             // CPU system-related metrics
             Self::SysCPUAvgIdle => "avg_idle".to_string(),

crates/obs/src/metrics/schema/scanner.rs

@@ -70,3 +70,12 @@ pub static SCANNER_LAST_ACTIVITY_SECONDS_MD: LazyLock<MetricDescriptor> = LazyLo
         subsystems::SCANNER,
     )
 });
+
+pub static SCANNER_ACTIVE_PATHS_MD: LazyLock<MetricDescriptor> = LazyLock::new(|| {
+    new_gauge_md(
+        MetricName::ScannerActivePaths,
+        "Current number of scanner paths being processed.",
+        &[],
+        subsystems::SCANNER,
+    )
+});