printing: Add metrics to track number of lacros extension printers

Metric name: Printing.LacrosExtensions.FromAsh.NumberOfPrinters.

When lacros is enabled, extension printing from ash side will go through
a mojom service to access lacros's extension printers.

Getting extension printers may be only called at most once when print
preview is opened. So this histogram wouldn't be flooded with entries.

Bug: b:40273973
Change-Id: Ie21a80d062fec4e29a8f1022f8735546a7db5c71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5601411


Commit-Queue: Xiangdong Kong <xiangdongkong@google.com>
Reviewed-by: Gavin Williams <gavinwill@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313167}