[jmx-metrics] Collect in callback

jmx-metrics/README.md

@@ -128,6 +128,11 @@ mutually exclusive with `otel.jmx.groovy.script`. The currently supported target
     [`CompositeData`](https://docs.oracle.com/javase/7/docs/api/javax/management/openmbean/CompositeData.html) instances,
     each key of their `CompositeType` `keySet` will be `.`-appended to the specified `instrumentName`, whose resulting
     instrument will be updated for each respective value.
+  - If the underlying MBean(s) held by the provided MBeanHelper are a mixed set of
+    [`CompositeData`](https://docs.oracle.com/javase/7/docs/api/javax/management/openmbean/CompositeData.html) instances
+    and simple values, the InstrumentHelper will not attempt to collect the metric as it would generate
+    metrics identified with the `instrumentName` and also the `instrumentName` with the `keySet` `.`-appended,
+    which breaks OpenTelemetry metric conventions.
 
 `otel.instrument()` provides additional signatures to obtain and update the returned `InstrumentHelper`:
 

jmx-metrics/src/integrationTest/resources/script.groovy

@@ -17,11 +17,13 @@
 import io.opentelemetry.api.common.Attributes
 
 def loadMatches = otel.queryJmx("org.apache.cassandra.metrics:type=Storage,name=Load")
-def load = loadMatches.first()
+if (!loadMatches.isEmpty()) {
+  def load = loadMatches.first()
 
-def lvr = otel.longHistogram(
-        "cassandra.storage.load",
-        "Size, in bytes, of the on disk data size this node manages",
-        "By"
-        )
-lvr.record(load.Count, Attributes.builder().put("myKey", "myVal").build())
+  def lvr = otel.longHistogram(
+    "cassandra.storage.load",
+    "Size, in bytes, of the on disk data size this node manages",
+    "By"
+  )
+  lvr.record(load.Count, Attributes.builder().put("myKey", "myVal").build())
+}