Potential issue with Counter internal slice management

[bernata]

What did you do?

I created a PR to demonstrate the problem:
#1295

To summarize, there are 4 counters:

	c1 := (&Counter{}).With("a", "1", "b", "2", "c", "3", "d", "4")
	c2 := c1.With("e", "5").(*Counter)
	c3 := c2.With("f", "6", "g", "7").(*Counter)
	c4 := c2.With("h", "8").(*Counter)

The counters c3 and c4 share the same backing memory for the counter lvs slice. As a result, c4 will overwrite the contents of c3 lvs slice. So in the example above, at the end of the execution, c3 lvs internal slice has values: a 1 b 2 c 3 d 4 e 5 h 8 g 7

What did you expect?

The expectation I have is c3 lvs internal slice has values: a 1 b 2 c 3 d 4 e 5 f 6 g 7
I assumed it behaved a bit like context.WithValue which I think would have similar behavior in keeping the parent-child contexts free from overwrite.

What happened instead?

The c3 lvs internal slice has values: a 1 b 2 c 3 d 4 e 5 h 8 g 7 because c4 overwrote part of the c3 slice.