Updates jmxfetch configs readme (#8769)

The script `copy-metric-configs.sh` has been replaced by a gradle task. And the github query is more precise.

Author: bric3

dd-java-agent/agent-jmxfetch/src/main/resources/datadog/trace/agent/jmxfetch/README

@@ -0,0 +1,9 @@
+# Metric Configs
+
+Files from [integrations-core](https://github.com/search?q=repo%3ADataDog%2Fintegrations-core+%22jmx_metrics%3A%22+language%3AYAML&type=code)
+are copied here at build time by the `copyMetricConfigs` gradle task after initializing the submodule.
+
+These are then bundled in `dd-java-agent.jar`. Due to limitations in Java jar walking, it is non-trivial
+to get all these files from within the jar without knowing their names.
+Consequently, we list out each integration in `datadog/trace/agent/jmxfetch/metricconfigs.txt` 
+so the agent can reference them.