fix(elasticsearch-logger): guard os.date against invalid index template

[AlinsRan]

TL;DR

Dynamic Elasticsearch index names containing {*t} / {!*t} produced a garbage index like services-table: 0x7f... instead of being rejected.

Cause

The index field supports {time_format} placeholders passed straight to os.date. The existing if not time guard assumes bad input returns nil, but on LuaJIT/OpenResty it never does:

• {*t} / {!*t} make os.date return a table, not a string. It then flows into ngx.re.gsub, which stringifies it into the index name.

Fix

Require os.date to return a string before using it; otherwise log failed to parse time format and fall back to an empty replacement. Wrapped in pcall for defense-in-depth on non-LuaJIT runtimes (where os.date can raise on a bad escape).

local ok, time = pcall(os_date, time_format)
if not ok or type(time) ~= "string" then
    core.log.error("failed to parse time format: ", time_format)
    return ""
end

Test

t/plugin/elasticsearch-logger2.t TEST 10: prefix{*t}suffix (and {!*t}) now resolves to prefixsuffix with a failed to parse time format log, instead of embedding a table: 0x... address.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change (N/A)
• I have verified that the changes pass the existing tests