Update metrics-web + metrics-lib to support Onionperf filters

We now have several new fields in the Onionperf analysis files, some of which will require introducing into metrics-web if we want to graph results for S61 instances further down the line.

Out of these, the most imporant is 'filtered_out', an optional boolean field that indicates whether the circuit is to be included or not in an Onionperf visualisation. A circuit can be filtered for a variety of reasons, one of them being whether the CBT was set or not at the time of measurement. This is the filter we will be using for S61 Onionperf analysis files.

What we need from metrics-web is that it graphs results only for the circuits that are not filtered (where the CBT was not set). This means it should discard tgen results where the circuit is either not known or 'filtered_out'. This will not impact any of the existing instances (they do not use filters)

It looks like metrics-lib is used to convert Onionperf analysis files that and the result is then used by metrics-web to construct its database.

I'm not familiar enough with how metrics-lib and metrics-web operate, but it looks like the field needs to be added to the both the converter (in metrics-lib) and the code that constructs the database (in metrics-web), however I'm not sure where the logic goes for including only certain results depending whether filtered_out is used.