stats: histogram, gauge and ring buffer types

stats: histogram, gauge and ring buffer types

A new log2 histogram type with prometheus exporter support.
min_exp can be set in the first element to adjust the bins.

The ring buffer is intended for exporting records to a client side
reader. If the reader cannot keep up the writer will overwrite oldest
entry.

Added a new gauge type, which is like scalar index, but being
explicit allows the prometheus exported to set type corretly.

Type: improvement
Change-Id: Ibe1244f28e01eee8d61a3ca6edb6fd1801f1c942
Signed-off-by: Ole Troan <[email protected]>
Change-Id: I1a6046c6962d67db8c510a571e9414723acbbd7e
Signed-off-by: Ole Troan <[email protected]>