docs(x): add readme

Author: MarcoPolo

x/README.md

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+# libp2p `x` packages
+
+The packages in this directory are experimental and subject to change or
+removal. They may introduce breaking changes without a major version bump. If
+you find these packages helpful, please let us know and we can work together on
+stabilizing them.
+
+## libp2p/x/metricsserver
+
+This is an experimental libp2p service that exposes a libp2p node's metrics to
+a set of allowlisted peers (by their Peer ID).
+