flag indicating whether any logs were discarded when capturing error_log using 'lua_capture_error_log' directive

[hachi029]

The error_log module just throw away the oldest ones silently if the user reads the buffered error log not fast enough or the buffer configured by lua_capture_error_log is too small, resulting in unnoticed logs lost.

There should be an api to read or return a flag indicating whether any logs were discarded.

For example：

local errlog = require "ngx.errlog"

local res, err, discarded = errlog.get_logs(30)

if err then 
     ...
end

if discarded then 
    -- change filter_level (for example, from ngx.INFO to ngx.NOTICE)
    errlog.set_filter_level(log_level)
end

[hachi029]

Is there any advice?