Serverless Metrics graphs are missing top unit line

**Describe the problem**

On a Staging 23.1.8 Serverless cluster, on the Metrics graphs, the top unit line is missing:

Compare this to the Custom Chart graphs on a demo 23.1.8 Self Hosted cluster DB Console, the top unit line is displayed:

**To Reproduce**

1. In Staging, select a 23.1.8 Serverless cluster from the Clusters page, and click Metrics in the Monitoring section of the left side navigation. The Monitor SQL Activity is selected by default.
2. Select a time frame that has data
3. Notice that peaks of the data go above the last unit line.

**Expected behavior**
There should be a unit line at or above the peak of a graph, similar to the DB Console Custom Chart graphs.

**Environment:**
- CockroachDB version: 23.1.8
- Server OS: Serverless
- Client app: CC Console

Jira issue: CRDB-31547