class SelectiveTypedProtoDecoder

TypedProtoDecoder with selective decoding. Ids allowlisted in the mask are

decoded into the O(1) dense storage as usual; every other field --

including ids beyond MAX_FIELD_ID, which TypedProtoDecoder drops -- is

appended, in wire order, to a spill area and can be iterated via

unknown_fields(). Spilled fields are not visible to Get()/at() (which

return an invalid field) or GetRepeated().

This is a separate subclass rather than an extra TypedProtoDecoder

constructor so that the spill bookkeeping costs the plain decode path

nothing: decoders are constructed per nested message, and even

zero-initializing one extra pointer member per construction is measurable

end-to-end.

Usage:

static constexpr SelectiveDecodeMask

<TracePacket

::kTimestampFieldNumber,

TracePacket::kInternedDataFieldNumber>

kMask{};

SelectiveTypedProtoDecoder

<TypedProtoDecoder

<MAX

_FIELD_ID>> decoder(

data, size, kMask);

for (const Field

&

f : decoder.unknown_fields()) { ... }