Fix issue with JobMetadata not longer supporting __setitem__ method

[VMRuiz]

Fixes https://github.com/scrapinghub/spidermon/issues/403

I did my best to test it locally, but I was not able to test it in a real Scrapy Cloud enviroment.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +1.14 :tada:

Comparison is base (e697dbe) 76.37% compared to head (3c7627e) 77.52%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #404 +/- ## ========================================== + Coverage 76.37% 77.52% +1.14% ========================================== Files 73 73 Lines 3103 3101 -2 Branches 518 516 -2 ========================================== + Hits 2370 2404 +34 + Misses 663 626 -37 - Partials 70 71 +1 ``` | [Impacted Files](https://app.codecov.io/gh/scrapinghub/spidermon/pull/404?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub) | Coverage Δ | | |---|---|---| | [spidermon/contrib/actions/jobs/tags.py](https://app.codecov.io/gh/scrapinghub/spidermon/pull/404?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub#diff-c3BpZGVybW9uL2NvbnRyaWIvYWN0aW9ucy9qb2JzL3RhZ3MucHk=) | `97.05% <100.00%> (+97.05%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/scrapinghub/spidermon/pull/404/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub)

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[Gallaecio]

I did my best to test it locally, but I was not able to test it in a real Scrapy Cloud enviroment.

While the code looks good to me, it may be best to get a manual test on Scrapy Cloud before merging.

[VMRuiz]

I did my best to test it locally, but I was not able to test it in a real Scrapy Cloud enviroment.

While the code looks good to me, it may be best to get a manual test on Scrapy Cloud before merging.

I forgot this is a public repo :sweat_smile:. I was able to test it, works as expected.