[pdata] `Map.Remove` mutates value returned by `Map.Get`

[dmitryax]

Map.Delete doesn't work as a clean map interface and mutates values returned by Map.Get. The following snippet

	m := pcommon.NewMap()
	m.InsertString("k1", "v1")
	m.InsertString("k2", "v2")
	v1, _ := m.Get("k1")
	fmt.Println(v1.StringVal())
	m.Remove("k1")
	fmt.Println(v1.StringVal())

prints

v1
v2

instead of expected

v1
v1

[dmitryax]

@bogdandrutu I believe this is a side-affect of this change https://github.com/open-telemetry/opentelemetry-collector/pull/2017 . Not sure if it warrants bringing back the following Map representation

type Map struct {
	orig *[]*otlpcommon.KeyValue
}

I would say we should. WDYT?

[bogdandrutu]

This is true for other slices, and I think we specifically told (was there at one point at least if I remember) in the description that instances returned by At and Get will be invalid if Remove or even add is called.

[bogdandrutu]

@dmitryax without that optimization we spend lots and lots of CPU allocating 1000s of small objects :))

[tigrannajaryan]

BTW, this and other optimization won't be necessary and won't cause problems like this if we move to https://github.com/splunk/exp-lazyproto :-)

[dmitryax]

Removing from 1.0 milestone since fixing it is not a breaking change for the pdata API