[configgrpc] IncludeMetadata is experimental

[TylerHelmuth]

From configgrpc.go

// Include propagates the incoming connection's metadata to downstream consumers.
// Experimental: *NOTE* this option is subject to change or removal in the future.
IncludeMetadata bool `mapstructure:"include_metadata"`

This issue tracks any work needed specifically for configgrpc, but the problem spans both confighttp and configgrpc in similar ways. For the sake of centralized discussion, lets start in #9381 and move here only if there are configgrpc-specific concerns.