Fix Field names containing '_items' crash field coverage

[VMRuiz]

Fixes https://github.com/scrapinghub/spidermon/issues/428

It only cover the case where the fields ends with "_items" but is prefixed with something else like "count_items". Using only _items as field name is still not compatible with the coverage count. We may be able to make it work but it would require a significant rewrite of calculate_field_coverage

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.43%. Comparing base (6b3503b) to head (957104a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #437 +/- ## ======================================= Coverage 79.43% 79.43% ======================================= Files 76 76 Lines 3229 3229 Branches 536 536 ======================================= Hits 2565 2565 Misses 593 593 Partials 71 71 ```

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