feat(agent): read the server each 10 seconds and send the peaks of the minute

[nabil1440]

Closes #41.

Part of the monitoring agent stack. This PR is on top of #40. Contract v0.4.0, "The readings" and "The peaks".

What changes

• Readings each 10 seconds. The agent loop now also wakes at the offset second plus 10, 20, 30, 40 and 50 seconds. At these times it takes a reading. It still sends one sample for each minute.
• Windows. The sample of the tick at T uses the windows from one reading to the next, from the tick before to T. If a reading fails, the two windows on each side of it join into one. A reading less than 5 seconds from its neighbours is left out, so that no window is some milliseconds long.
• New optional sample fields: cpu_max_percent, memory_used_max_bytes, swap_used_max_bytes, net_in_max_bytes_per_second and net_out_max_bytes_per_second. A value that is not known is null.
• Guarantees. cpu_percent ≤ cpu_max_percent, memory_used_bytes ≤ memory_used_max_bytes ≤ memory_total_bytes, the same for swap, and net_in_bytes ≤ 60 × net_in_max_bytes_per_second up to rounding.
• Network peaks are null when net_counters_reset is true, or when a counter of a window went back.
• Real reading times. Each reading carries the time at which it ran, with the monotonic clock. A late timer or a step of the wall clock does not change the length of a window. recorded_at stays the time of the tick.
• A slow reading does not skip the tick after it.
• apt-check runs after the reading of the tick, so that it does not move the reading.
• The fields of contract v0.3.1 do not change. The alerts read them as before.
• A short first minute has no sample. After a fresh start (no recent saved counters), when the first tick comes less than 10 seconds after the start, the agent sends no sample for that minute, and logs it at Info. The tick reading starts the next minute, which has all its values. Before, the first sample after an install could show 100% CPU from the load of the install.

Before release

• A control plane that implements contract v0.3.1 must accept a sample with the new fields: the contract says that it ignores a field that it does not know. Checked: the request data class of the control plane accepts a report with all the fields of v0.4.0 and v0.5.0.

Tests

• Loop (fake clock): the readings at the correct seconds, no reading at a tick, and a reading of 10 seconds does not skip the tick.
• Collector: the peaks of each field, a failed reading, the first sample after a start, a restart with a saved reading, a counter that went back, readings that are not newer, the limit of 30 readings, and a failed tick (the peaks come from the last minute only).
• A sample queued by an older agent sends the peaks as null.
• make check passes.