[RFE] add running eBPF program metrics

[natoscott]

Statistics relating to the performance overheads from running eBPF programs are available in modern kernels via the bpf(2) syscall:

 * BPF_ENABLE_STATS
 *	Description
 *		Enable eBPF runtime statistics gathering.
 *
 *		Runtime statistics gathering for the eBPF runtime is disabled
 *		by default to minimize the corresponding performance overhead.
 *		This command enables statistics globally.
 *
 *		Multiple programs may independently enable statistics.
 *		After gathering the desired statistics, eBPF runtime statistics
 *		may be disabled again by calling **close**\ (2) for the file
 *		descriptor returned by this function. Statistics will only be
 *		disabled system-wide when all outstanding file descriptors
 *		returned by prior calls for this subcommand are closed.
 * 

It'd be great to make these available as metrics from the BPF PMDA for reporting, recording and visualizing via PCP tools and grafana-pcp.