PMM-15161 False auto increment.

[JiriCtvrtka]

PMM-15161

FB: Percona-Lab/pmm-submodules#4504

Problem

The max_int expression in infoSchemaAutoIncrementQuery detects unsigned
columns with an anchored pattern:
end + (column_type like '% unsigned') - 1
ZEROFILL appends a further attribute after unsigned, so column_type is e.g. int unsigned zerofill and the LIKE '% unsigned' no longer matches. Such columns get the signed bit count, so mysql_info_schema_auto_increment_column_max is reported as half of the real limit (e.g. 2^31-1 instead of 2^32-1) — the column looks twice as full as it is, and any dashboard/alert built on auto_increment / max fires far too early.

Fix

Match unsigned anywhere in column_type:
end + (column_type like '%unsigned%') - 1
unsigned only ever appears in column_type as the numeric attribute, so the
unanchored pattern cannot produce a false positive.

Tests

The collector had no test coverage at all, so this adds both levels:

collector/info_schema_auto_increment_test.go — sqlmock unit test covering the rows → metrics mapping (both metrics per row, labels, gauge type, no extra metrics, expectations met).
collector/info_schema_auto_increment_live_test.go — runs the real query against the server started by docker-compose, so every flavor of the CI database matrix verifies the max_int expression. It creates one table per case — signed int, int unsigned, int unsigned zerofill, smallint unsigned, bigint unsigned — and asserts the exact expected maximum. Skipped under -short.

[JiriCtvrtka]

@copilot review

[JiriCtvrtka]

@copilot review

[JiriCtvrtka]

@copilot review