Sentinel for dependencies_metadata / objects_metadata on streams that
don’t use the transit/POD wire format (e.g. OTLP). Existing readers decode
these BYTEA columns as Vec<UserDefinedType> and iterate; an empty Vec
makes those loops no-ops without touching consumer code.
Drops every client-supplied property whose key starts with the reserved micromegas.
namespace (RESERVED_PROPERTY_PREFIX), so that namespace can never be asserted from the
payload. A dropped key is warn!-logged once per call, naming the key – a native client
setting e.g. micromegas.audience is either attempting a client-side self-stamp or probing,
and both are worth seeing. Stripping rather than rejecting the request with 400 keeps such a
producer’s telemetry flowing, even though the self-stamp attempt has no effect.