Currently classed PlanStageData and RuntimeEnvironment have debugString() methods:

https://github.com/mongodb/mongo/blame/2ce7d457434f2b81924c32ead1fefce7d9e3a949/src/mongo/db/query/sbe_stage_builder_plan_data.h#L299

https://github.com/mongodb/mongo/blob/master/src/mongo/db/exec/sbe/expressions/runtime_environment.h#L193

However, they have different signatures:

std::string debugString() and 

void debugString(StringBuilder*).

RuntimeEnvironment is accepting StringBuilder to avoid extra allocations when serializing it in PlanStageData::debugString().

What we should do is:

1. Add serialize(fmt::memory_buffer&) method to both functions. This way they can be serialized to already existing buffer, avoiding allocations and this signature is supported by LOGV2: https://github.com/mongodb/mongo/blob/master/src/mongo/logv2/attribute_storage.h#L173
2. Add std::string debugString() to both classes, implementing it via serialize(). This way we can keep old signature.