Implementing "size" method for Accumulator

[MichaelScofield]

What type of enhancement is this?

Performance

What does the enhancement do?

The size method in DataFusion's Accumulator is important for some statistics and performance optimization (like "size hint"). See its doc:

/// Allocated size required for this accumulator, in bytes, including `Self`.
/// Allocated means that for internal containers such as `Vec`, the `capacity` should be used
/// not the `len`

Currently it's always 0.

Implementation challenges

No response