Explore mTLS request observability support

From HN thread:

> Speaking of observability tools. Anybody here know how to gather more in-depth metrics on mTLS requests? Have an internal (self signed) CA and just want to know which issued certs are presented to nodes. Would be nice to get cert serial number and other metadata as well

This is an interesting ask that can help with basic visibility into mTLS issues. Explore how can this be solved and what TLS funcs can be plugged into with eBPF to solve this issue
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