Add description field to LabelKey

[rghetia]

This fixes #1063.

This is a breaking change. The only user of the LabelKey is stackdriver exporter and prometheus exporter. Prometheus exporter is handled as part of this commit. PR for SD change is opened.

[songy23]

Please fix CI.

[songy23]

Consider leaving Description empty.

[rghetia]

ok

[rghetia]

Please fix CI.

@songy23, I don't think failure is related to the change. can you please confirm?

[songy23]

# runtime
C:\go\src\runtime\cpuflags_amd64.go:14:2: offsetX86HasAVX2 redeclared in this block
	previous declaration at C:\go\src\runtime\cpuflags.go:14:37
C:\go\src\runtime\mem_windows.go:27:6: sysAlloc redeclared in this block
	previous declaration at C:\go\src\runtime\mem_aix.go:14:43
C:\go\src\runtime\mem_windows.go:32:6: sysUnused redeclared in this block
	previous declaration at C:\go\src\runtime\mem_aix.go:32:36
C:\go\src\runtime\mem_windows.go:62:6: sysUsed redeclared in this block
	previous declaration at C:\go\src\runtime\mem_aix.go:36:34
C:\go\src\runtime\mem_windows.go:87:6: sysFree redeclared in this block
	previous declaration at C:\go\src\runtime\mem_aix.go:42:52
C:\go\src\runtime\mem_windows.go:96:6: sysFault redeclared in this block
	previous declaration at C:\go\src\runtime\mem_aix.go:48:35
C:\go\src\runtime\mem_windows.go:101:6: sysReserve redeclared in this block
	previous declaration at C:\go\src\runtime\mem_aix.go:52:46
C:\go\src\runtime\mem_windows.go:114:6: sysMap redeclared in this block
	previous declaration at C:\go\src\runtime\mem_aix.go:60:51
C:\go\src\runtime\netpoll_windows.go:36:6: netpollinit redeclared in this block
	previous declaration at C:\go\src\runtime\netpoll_aix.go:55:6
C:\go\src\runtime\netpoll_windows.go:44:6: netpolldescriptor redeclared in this block
	previous declaration at C:\go\src\runtime\netpoll_aix.go:90:26
C:\go\src\runtime\netpoll_windows.go:44:6: too many errors

Seems to be related to the Windows host.