For some reason we've disabled usage of json formatter in early
logging since the commit 098324556. Lets allow it back.

v2:
 - add early verification of options for cases like
   box.cfg{log="syslog:", log_format="json"}
 - extend test

v3:
 - pass the whole cfg table into verify_option helper
 - use string.startswith helper in verify_format

branch gorcunov/gh-5121-logger-boot-json-3
issue https://github.com/tarantool/tarantool/issues/5121

Cyrill Gorcunov (3):
  core/say: allow to use json in boot logger
  lua/log: allow to use json formatter early
  test: app-tap/logger -- test json in boottime logger

 src/box/lua/load_cfg.lua     |  6 +++---
 src/lib/core/say.c           |  7 ++----
 src/lua/log.lua              | 42 +++++++++++++++++++++++++++---------
 test/app-tap/logger.test.lua | 19 ++++++++++++----
 4 files changed, 52 insertions(+), 22 deletions(-)

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2.26.2