[common] add PerformanceTimer

[a114j0y]

What I did

Extend the PerformanceIntervalTimer utility in sonic-sairedis repo to measure API performance.

To optimize performance of a periodically invoked function, we not only pay attention to how long it takes to finish execution (busy time), but also care about how long it waits to get executed (idle time). The total time determines the actual throughput in the real world system.

Hence we want to see data in 3 ways:

1. Idle time between two calls (a gap between this start and the last end, during which the thread just waits)
2. Busy time of each API call (interval between this start and this end, it's the true execution time)
3. Total time elapsed (idle + busy)

| <- Initial Gap -> | <- - 1st **Execution** - -> | <- 2nd Gap -> | <- - 2nd **Execution** - -> |

| <---------------- 1st **Total** --------------> | <------------ 2nd **Total**--------------> |

Other features

• calculate the number of tasks finished per call (Routes Per Second in the context of routes loading)
• batch the logging output
• adjust the SWSSLOGLEVEL using a file indicator, default level is "INFO", hence we could enable perf logs in a dynamic way.

Why I did it

• original utility has limited functionality

  • we need gap data to obtain more insight on the mutual influence of upstream and downstream modules

• original utility is in libsairedis, to use it in sonic-swss, there would be much Makefile.am change, not necessary.

• a utility tool for swss should be included in swss-common

• when enabled on-demand, it could help measure the API performance under scaled traffic

  • one use case: help measuring perf in pr optimized bgp loading speed