[MONITORING][FEATURE] Mercury PVAR integration to tease out fine-grained event contribution to RPC runtime

[carns]

Note that this issue will require corresponding PVAR support in Mercury to implemented as a prerequisite.

• Mercury PVARs are needed to “fill in the gaps” in instrumentation.
• Export necessary PVARs from Mercury.
• Initialize PVAR session in Margo.
• Sample PVARs at “appropriate” location:
• PVAR data either gets added to breadcrumb profile and/or trace event.

[mdorier]

Edited the title, I think it's still relevant in the context of the new monitoring system.