Use CopyOnWritePropertyDictionary.ImportProperties for batching

[rokonec]

Fixes #8673

Context

High memory allocations in CopyOnWritePropertyDictionary ImmutableDictionary was reported by partners.

Changes Made

Use CopyOnWritePropertyDictionary.ImportProperties in obvious places as oppose to CopyOnWritePropertyDictionary.Set in loop.

Testing

Unit tests. Local. Perf measure.

Notes

[JanKrivanek]

Overal looks good. I have just 2 minor points for consideration

[ladipro]

super-nit: Would it make sense to add a constructor to CopyOnWritePropertyDictionary<T> taking IEnumerable<T>?

[rokonec]

@ladipro

Would it make sense to add a constructor to CopyOnWritePropertyDictionary taking IEnumerable

It would make sense for code readability, but since the constructor CopyOnWritePropertyDictionary<T> use static Empty pattern, it should result in same amount of allocations as current code. I was trying not to change API of CopyOnWritePropertyDictionary.

[danmoseley]

Is Select and delegate here as efficient as a plain loop?

The core libraries would not use this, I think (@stephentoub?)

[stephentoub]

Is Select and delegate here as efficient as a plain loop?

It depends on several factors, including what DirectMetadata is.

The core libraries would not use this, I think (@stephentoub?)

It also depends :) but generally we avoid use of IEnumerable<T> manipulation on hot paths.

[rokonec]

Purpose of this changes was to eliminate generation of ImmutableDictionary in a loop which has very very bad CPU and memory allocation consequences.
To do that I decided to use prior existing internal void ImportProperties(IEnumerable<T> other) method which calls public ImmutableDictionary<TKey, TValue> SetItems(IEnumerable<KeyValuePair<TKey, TValue>> items) internally.
I do not see how to avoid IEnumerable<T> here, unless we would change CopyOnWritePropertyDictionary API to somehow expose immutable builder. I don't think this path is that hot though.