Proposal: Add sync.Map.Len() method

[andeya]

I suggest adding the Len method to the version of sync.Map in the Go1.9 version.

func (m *Map) Len() int

The following is a reference:

https://github.com/henrylee2cn/goutil/blob/master/map.go#L604

[linlinjie]

Very practical，I ding you!

[egonelbre]

Duplicate of #20680. Other method suggestions in 1. #21199.

[xianlezheng]

or Size()

[ianlancetaylor]

Closing as duplicate.