Allow specifying alerts on metrics different than CPU

Currently, when specifying the alert threshold e.g. `alert-threshold: '200%'` we cannot specify the metric which we want to alert on.

For instance Go toolchain reports the following metrics:

- ns per benchmark
- Bytes allocated per benchmark
- allocations per benchmark

This action compares the first metric - ns per benchmark - which might be flaky e.g. on some CI systems.

This issue is a feature request to allow alerts on different metrics.