proposal: sync: add sync.Map.Len()

[mattn]

Currently, no fast way to get length of sync.Map. To get the length, we should use Range with iterating elements. Hopefully to add Len() to get the length.

// Len returns a size of the map.
func (m *Map) Len() int {
	if m.dirty != nil {
		return len(m.dirty)
	}
	read := m.loadReadOnly()
	return len(read.m)
}

[mattn]

func (m *DeepCopyMap) Len() int {
	clean, _ := m.clean.Load().(map[any]any)
	return len(clean)
}

func (m *RWMutexMap) Len() int {
	m.mu.RLock()
	l := len(m.dirty)
	m.mu.RUnlock()
	return l
}

cpu: AMD Ryzen 5 3500U with Radeon Vega Mobile Gfx
BenchmarkLen/*sync_test.DeepCopyMap-8           645149230                2.034 ns/op
BenchmarkLen/*sync_test.RWMutexMap-8            24492294                44.83 ns/op
BenchmarkLen/*sync.Map-8                        1000000000               1.009 ns/op

[dsnet]

See #20680. In particular the comment here: #20680 (comment)

[mvdan]

This proposal adds no new information, so I'm closing it as a duplicate for now.