Step 22 C: metrics over the registry, documentation, PR-doc

The metrics page sums the registered repositories' sizes; the instance
configuration is documented in docs/configuration.md, the architecture skill,
AGENTS.md and the step 22 plan (session C ticked, fairness decided FIFO).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/main/kotlin/de/hoennig/werkator/metrics/MetricsConfiguration.kt

@@ -1,6 +1,7 @@
 package de.hoennig.werkator.metrics
 
 import de.hoennig.werkator.build.ArtifactStore
+import de.hoennig.werkator.repo.RepoRegistry
 import org.springframework.context.annotation.Bean
 import org.springframework.context.annotation.Configuration
 import java.time.Clock
@@ -16,10 +17,12 @@ class MetricsConfiguration {
     @Bean
     fun systemMetricsCollector(
         artifactStore: ArtifactStore,
+        registry: RepoRegistry,
         clock: Clock,
     ): SystemMetricsCollector =
         SystemMetricsCollector(
             stateFile = { artifactStore.rootDir().resolve(SystemMetricsCollector.STATE_FILE_NAME) },
+            repoDirs = { registry.all().map { it.workingDir } },
             clock = clock,
         )
 }

src/main/kotlin/de/hoennig/werkator/metrics/SystemMetricsCollector.kt

@@ -35,7 +35,8 @@ data class PersistedMetricsState(
  */
 class SystemMetricsCollector(
     private val stateFile: () -> Path,
-    private val workingDir: Path = Paths.get("."),
+    /** The served repositories: the disk metric is the first one's file store, the repository size their sum. */
+    private val repoDirs: () -> List<Path> = { listOf(Paths.get(".")) },
     private val clock: Clock = Clock.systemUTC(),
     private val procStat: Path = Paths.get("/proc/stat"),
     private val procMeminfo: Path = Paths.get("/proc/meminfo"),
@@ -239,7 +240,7 @@ class SystemMetricsCollector(
             .removeSuffix(" kB")
             .toLong()
 
-    private fun readDisk(): DiskSpace? = readSource("disk") { diskSpace(workingDir) }
+    private fun readDisk(): DiskSpace? = readSource("disk") { diskSpace(repoDirs().first()) }
 
     /**
      * The repository size is expensive to determine (a full file walk), so unlike
@@ -251,7 +252,7 @@ class SystemMetricsCollector(
             samplesSinceRepoSizeProbe++
             return lastRepoSizeGib
         }
-        lastRepoSizeGib = readSource("repo size") { repoSizeBytes(workingDir) / BYTES_PER_GIB }
+        lastRepoSizeGib = readSource("repo size") { repoDirs().sumOf(repoSizeBytes) / BYTES_PER_GIB }
         samplesSinceRepoSizeProbe = 1
         return lastRepoSizeGib
     }