ui: add charts on gc assist metric

[lyang24]

This commit adds a line chart contains gc assist duration on the runtime page. The goal is to present the estimated time user go routines spend on assisting gc tasks.

Informs: https://github.com/cockroachdb/cockroach/pull/118875

Release note: None

gc assist on crdb single node with kv workload gcttl=5s for database in (kv, system)

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[cockroach-teamcity]

This change is 

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Thank you for updating your pull request.

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[lyang24]

bors r+

[craig[bot]]

Build succeeded:

• Bazel Essential CI (Cockroach)