Avoid rounding error in `test_prometheus_collect_count_total_by_cost_multipliers`

[hendrikmakait]

Closes #xxxx

• [ ] Tests added / passed
• [ ] Passes pre-commit run --all-files

[github-actions[bot]]

Unit Test Results

_See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests._

    29 files  ±    0      29 suites  ±0   11h 3m 45s :stopwatch: + 1h 17m 43s  4 058 tests  -     5   3 957 :white_check_mark: +    8     97 :zzz:  -   9  4 :x: ±0  55 897 runs  +7 591  53 729 :white_check_mark: +7 359  2 163 :zzz: +249  5 :x: ±0 

For more details on these failures, see this check.

Results for commit f08f0428. ± Comparison against base commit 9672121c.

This pull request removes 13 and adds 8 tests. Note that renamed tests count towards both.

``` distributed.protocol.tests.test_arrow distributed.protocol.tests.test_collection distributed.protocol.tests.test_highlevelgraph distributed.protocol.tests.test_numpy distributed.protocol.tests.test_pandas distributed.shuffle.tests.test_graph distributed.shuffle.tests.test_merge distributed.shuffle.tests.test_merge_column_and_index distributed.shuffle.tests.test_metrics distributed.shuffle.tests.test_rechunk … ``` ``` distributed.diagnostics.tests.test_memray ‑ test_basic_integration_scheduler distributed.diagnostics.tests.test_memray ‑ test_basic_integration_scheduler_report_args[False] distributed.diagnostics.tests.test_memray ‑ test_basic_integration_scheduler_report_args[report_args0] distributed.diagnostics.tests.test_memray ‑ test_basic_integration_workers[1] distributed.diagnostics.tests.test_memray ‑ test_basic_integration_workers[False] distributed.diagnostics.tests.test_memray ‑ test_basic_integration_workers[True] distributed.diagnostics.tests.test_memray ‑ test_basic_integration_workers_report_args[False] distributed.diagnostics.tests.test_memray ‑ test_basic_integration_workers_report_args[report_args0] ```

:recycle: This comment has been updated with latest results.