LBR Source/Target Address Support

Hi! I found some old issues (ie #1234 from 2017) which mention that LBR support was on the BPF TODO list. Has this work been completed? If not, is there any tracking/discussion about it that I can follow or contribute to?

I can write a bpf program that watches for `BR_INST_RETIRED.NEAR_TAKEN` events, but I don't have a good way to actually access the source/target address ring buffer (doesn't look like I can read the `MSR_LASTBRANCH_*` registers directly from BPF, and `bpf_perf_event_read_value` just returns the pmu counter value, not the source/target values).

Start by reviewing the referenced issue #1234 and the BPF program approach using BR_INST_RETIRED.NEAR_TAKEN. Investigate how source and target values from the MSR_LASTBRANCH_* registers could be exposed, since bpf_perf_event_read_value only returns the PMU counter value. Done means documenting the current support status and identifying a concrete tracking or contribution path.

Written by the indexing model from the issue text.