Improve discarded metric

[alanshaw]

The provider record prefetch chart shows discarded requests in orange. These are currently the requests where we previously failed to find a CID and were asked again within 1 hour OR if the prefetch queue was full.

We should categorize these properly so that we have the following discarded categories:

• Previously failed to find a CID and were asked again within 5 minutes
• Previously failed to find a CID and were asked again within 1 hour
• Prefetch queue was full