metrics: add more detailed measurements for starting proxy service

[mkemel]

Please describe what you would like to see

Currently the metrics we report for a StartUnit job are:

1. Overall time spent on the job, measured on hirte manager
2. Time that the unit was in "activating" state, read from unit properties
3. Time it took for systemd job to run, measured on agent

But when starting a unit with dependency on another node (i.e. requiring proxy service), we can see quite a long time measured on the agent (3), and much shorter time spent in "activating" state.

Please describe the solution you'd like

We need higher resolution metrics on starting service with proxy.