Query plan cache metrics

[lennyburdette]

A customer asked for metrics related to the query plan cache, especially cache eviction rates.

They implemented custom metrics in Apollo Gateway while investigating performance issues but @BrynCooke indicated that these could be generally useful in the router (especially once we support external caches).

[abernix]

I'm going to close this in favor of #1985, which has query plan cache evictions as one of the proposed metrics. Feel free to opine over there if you have ideas for more! 🙏