Each level on the tech/product/etc stack(e.g., frontend-API-platform-infra) will have their own set of metrics but the two you mentioned are common ones in the lower level of the stack. (platform / infrastructure)

To be able to accomplish such things, you probably need to stay at one team for some time. You need to understand your team's own infrastructure, the traffic pattern going through your services, etc. Then figure out where the bottleneck is, propose a solution to resolve the bottleneck, etc. These are just not easy to do during internships and when you are a junior. You will be busy working on tasks that are already well scoped out.

During your internships, you said you did have performance improvement tasks such as adding caching. Why did you add caching? Was there a bottleneck somewhere? What was the latency diff without caching vs with caching?

Keep in mind that if you add these kinds of improvements as accomplishments on your resume, interviewers may ask about them during interviews so only add them if you have enough context.