merge: Simplify usage of ComputeMetricExporter

This pull request addresses some issues with the current implementation of 
the `ComputeMetricExporter` class. 
In particular, the construction of `ComputeMetricExporter` does not require a `Clock` anymore.

- Ensure shutdown of exporter is called 
- Do not require clock for ComputeMetricExporter
- Do not recover guests in non-error state
- Write null values explicitly in Parquet exporter
- Report cause of compute exporter failure

**Breaking API Changes**

- `ComputeMetricExporter` is now an abstract class that can be extended to collect metrics
- `ParquetComputeMonitor` has been renamed to `ParquetComputeMetricExporter` and extends `ComputeMetricExporter`

2 files changed, 15 insertions, 4 deletions

diff --git a/opendc-compute/opendc-compute-simulator/src/main/kotlin/org/opendc/compute/simulator/SimHost.kt b/opendc-compute/opendc-compute-simulator/src/main/kotlin/org/opendc/compute/simulator/SimHost.kt
index ff55c585..fdb3f1dc 100644
--- a/opendc-compute/opendc-compute-simulator/src/main/kotlin/org/opendc/compute/simulator/SimHost.kt
+++ b/opendc-compute/opendc-compute-simulator/src/main/kotlin/org/opendc/compute/simulator/SimHost.kt
@@ -262,7 +262,7 @@ public class SimHost(
     }
 
     override suspend fun delete(server: Server) {
-        val guest = guests.remove(server) ?: return
+        val guest = guests[server] ?: return
         guest.terminate()
     }
 
@@ -296,7 +296,7 @@ public class SimHost(
         _bootTime = clock.millis()
 
         for (guest in guests.values) {
-            guest.start()
+            guest.recover()
         }
     }
 
diff --git a/opendc-compute/opendc-compute-simulator/src/main/kotlin/org/opendc/compute/simulator/internal/Guest.kt b/opendc-compute/opendc-compute-simulator/src/main/kotlin/org/opendc/compute/simulator/internal/Guest.kt
index 90562e2f..7f33154a 100644
--- a/opendc-compute/opendc-compute-simulator/src/main/kotlin/org/opendc/compute/simulator/internal/Guest.kt
+++ b/opendc-compute/opendc-compute-simulator/src/main/kotlin/org/opendc/compute/simulator/internal/Guest.kt
@@ -96,8 +96,8 @@ internal class Guest(
             }
             ServerState.RUNNING -> return
             ServerState.DELETED -> {
-                logger.warn { "User tried to start terminated server" }
-                throw IllegalArgumentException("Server is terminated")
+                logger.warn { "User tried to start deleted server" }
+                throw IllegalArgumentException("Server is deleted")
             }
             else -> assert(false) { "Invalid state transition" }
         }
@@ -144,6 +144,17 @@ internal class Guest(
     }
 
     /**
+     * Recover the guest if it is in an error state.
+     */
+    suspend fun recover() {
+        if (state != ServerState.ERROR) {
+            return
+        }
+
+        doStart()
+    }
+
+    /**
      * The [Job] representing the current active virtual machine instance or `null` if no virtual machine is active.
      */
     private var job: Job? = null