Add support for single-value metrics

[smarr]

At the moment, we are recording per iteration metrics.

Though, some metrics are unchangeable between iterations, or independent of a specific invocation.

This would include things like:

• size of binary produced in build step
• other statistics like compilation time from build step, etc
• overall process-start to process-finish time
• maximum memory use for whole process, maxRSS (we already have some support for this, but not as a process-level metric)

@sophie-kaleba: any concrete examples we should consider?