Can't handle http-requests with a list of required metrics

[willardgibbs]

When additional metrics (#95) are calculated, necessary for calculating metrics are taken from CollectorRegistry. List of metrics names are passed from the result of the HTTP-query parsing to the filteredMetricFamilySamples method.
There is bug: necessary metrics may not be in the list, and HTTP-response will not be sent. Only necessary for calculations metrics names should be passed in method.