docs(*) document internal differences between SDK hosts

docs/PROXY_WASM_HOST_DIFFERENCES.md

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+# Proxy-Wasm Host Differences
+
+Listed here are noteworthy internal discrepancies (implementation differences)
+between ngx_wasm_module and other Proxy-Wasm host implementations.
+
+## Table of Contents
+
+- [Metrics Prefixing](#metrics-prefixing)
+
+## Metrics Prefixing
+
+- Envoy internally prefixes metric names with `wasmcustom.*` and only exposes
+  this prefix in the output of the `/metrics` endpoint.
+- For internal implementation reasons, ngx_wasm_module prefixes metric names
+  with `pw.[filter_name].*`, where `filter_name` is the name of the filter
+  that defined the metric. Only the ngx_wasm_module FFI library may expose the
+  existence of this prefix.
+
+Proxy-Wasm SDK users need not worry as metric names are never exposed through
+the SDK in the first place.
+
+[Back to TOC](#table-of-contents)