Fix batch implementation logic

[zippolyte]

What does this PR do?

Fixes the batch implementation logic. Before this fix, we didn't reset the query_spec object at each loop run, so we ended up doing total_nb_objects / batch_size requests, each querying metrics for every objects instead of just the objects of the batch.

Motivation

What inspired you to submit this pull request?

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo

Additional Notes

Anything else we should know when reviewing?

[codecov-io]

Codecov Report

Merging #2265 into master will decrease coverage by 4.48%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2265      +/-   ##
==========================================
- Coverage    86.1%   81.62%   -4.49%     
==========================================
  Files         189       15     -174     
  Lines       13639     1219   -12420     
  Branches     1442      134    -1308     
==========================================
- Hits        11744      995   -10749     
+ Misses       1488      193    -1295     
+ Partials      407       31     -376

[masci]

LGTM!