devserver: add back the /doc/ for /check_health

The old implementation of /doc of devserver doesn't work when we
refactor devserver functions into apps. This CL enables devserver to get
exposed methods from mounted apps.

BUG=chromium:993621
TEST=Started devserver locally, and ran below two commands:
  1. curl http://localhost:8080
  Verified that /check_health is listed.
  2. curl http://localhost:8080/doc/check_health
  Verified that the doc string of /check_health is displayed.

Change-Id: Iec8ad5890b655069fdf5cff17196a147e3b9e147
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/1763029
Reviewed-by: C Shapiro <shapiroc@chromium.org>
Tested-by: Congbin Guo <guocb@chromium.org>
Commit-Queue: Congbin Guo <guocb@chromium.org>
Auto-Submit: Congbin Guo <guocb@chromium.org>
1 file changed