Show gaps in the dashboard when the agent was not able to send data

[gabocic]

It happened to me while troubleshooting two different servers, that the memory graph was not truly reflecting that server was out of memory. @kochetovd mentioned that

The last RAM usage metric value was 15-53 and equal to 15 GB. Then the agent stopped sending data. However, the dashboard shows the average values for 10 minutes.

This was misleading as the host was running out of memory and mysqld was being OOM-killed.

My proposal is for the graphs to show a gap or some sort of visual warning to tell the user that the agent was unresponsive or that the data sample was not retrieved thus there might be missing values

[drupaladmin]

@gabocic @kochetovd We can add additional events to the timeline like Agent is not available.

[kochetovd]

I'll also add that the free command output was received when the agent became unavailable.
It would also be useful to display the timestamp of the last collected metrics.
This is because detecting that the agent is unavailable takes some time, and the event will appear slightly later than the actual end of metric collection.