Issue #140: Performance checks for metrics

[petersistrom]

Resolves #140

[petersistrom]

To test I ran the following code to insert data into the logs

GLOBAL $DB;
$dataobjects = [];
$res = (1661313653 - 1642996853) / 100;
for ($i = 1590465600; $i < 1661313653; $i += $res) {
    $dataobjects[] = [
        'eventname' => '\core\event\user_created',
        'component' => 'core',
        'action' => 'created',
        'target' => 'user',
        'crud' => 'r',
        'edulevel' => 0,
        'contextid' => 1,
        'contextlevel' => 10,
        'contextinstanceid' => 0,
        'userid' => $i,
        'anonymous' => 0,
        'timecreated' => $i
    ];  
}
$DB->insert_records('logstore_standard_log', $dataobjects);

Then backfilled Online Users for 2 months and the last value is displayed

[jaypha]

I think it should be more descriptive than just saying '1', like "Last recorded '1' at 12:55", or similar.

[brendanheywood]

This is intentional as this is going to get pushed elsewhere for consumption and I want it to be easy to cast to an int or float. So just put that in a code comment