Wasm Upstream Http Wasm Stats Use Root Scope

Upstream HTTP Wasm filter stats now use the server-wide root stats scope instead of the cluster stats scope, matching downstream HTTP Wasm filter scoping. For a custom counter foo in the wasmcustom namespace, admin /stats output changes from cluster.<cluster_name>.wasmcustom.foo to wasmcustom.foo. Prometheus output changes from envoy_cluster_wasmcustom_foo{envoy_cluster_name="X"} to foo because the leading registered custom namespace is stripped by the Prometheus formatter. Plugins that need per-cluster differentiation should encode the cluster name into the metric name themselves. Can be reverted by setting runtime guard envoy.reloadable_features.upstream_wasm_filter_uses_root_scope to false.