Handle duplicated metrics gracefully

Summary:

The exporter remembers metrics by Graphite name. It is possible to get into situations where multiple samples (graphite metrics) result in the same Prometheus metric.

At the moment, this makes the exporter catastrophically fail (return error 500 on /metrics). Instead, it should discard the older sample and use the new sample's value.

Original issue description:

We tried using v0.5.0 in our dev env today, upgrading from 0.4.2.
After the upgrade, metrics seem to be dropping out regularly, as can be clearly seen in this grafana dashboard.

So something is definitively wrong here.
There are no changes to the environment or config between 0.4.2 and 0.5.0, and we can not see anything abnormal in the graphite_exporter logfiles.

This looks like a critical bug to us.

It should be noted that we have seen similar behavior before where we had UDP packet drops, but this was resolved by tuning the UDP receive buffer as explained here: https://nav.uninett.no/doc/4.3/howto/tuning_graphite.html

Start at the exporter’s /metrics endpoint and trace where Graphite names become Prometheus metrics. Check the existing tests for duplicate samples; done means repeated names no longer cause a 500 response and the newer sample is exported.