.gitlab-ci.yml: add partial flushing benchmark to the benchmarking platform

[katiehockman]

What does this PR do?

Adds the new partial flushing benchmark to the benchmarking platform.

Motivation

The feature branch has now been merged into main.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-08-03 19:39:22

Comparing candidate commit e031d3f in PR branch katie.hockman/partial-flush-bench with baseline commit 2fffe7b in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 40 metrics, 1 unstable metrics.