duplicated tests on CollecTor metrics

AFAIK bridge is being tested every 18h and currently we add a line in the CollecTor metrics for each test. That means that we produce two lines for some bridges each day (there is a bug where we produce more that two #23). Each test line has only the test result and the bridge hash, so the duplicated lines are not so useful as you can't know which was produced before.

Do we just care about the last test and should only report one line per bridge? It has being useful to find out issues (#20 (closed)) to have the full list of tests, but maybe once we finish fixing the mayor issues we don't really need the result of each test.

If we want to keep reporting each test maybe we want to report the order they had happen, by using a monotonic clock or any other counting mechanism. Or add a timestamp so we can use it to know when it did happen. If we go for a timestamp we might want to do some simple obfuscation, like truncate it to hourly resolution.