Peering metrics exporting routines generates concurrent map R/W operations

[cortze]

The metrics exporting service of the Peering-Pruning module produces a FATAL error. The error:

fatal error: concurrent map iteration and map write

goroutine 140 [running]:
runtime.throw({0xdeca2c, 0xd762a0})
    /usr/local/go/src/runtime/panic.go:1198 +0x71 fp=0xc002c3acd8 sp=0xc002c3aca8 pc=0x437b11
runtime.mapiternext(0xc002c3ae60)
    /usr/local/go/src/runtime/map.go:858 +0x4eb fp=0xc002c3ad48 sp=0xc002c3acd8 pc=0x412bcb
github.com/migalabs/armiarma/src/peering.(*PeeringService).ServeMetrics.func1()
    /home/cortze/cortze/armiarma/src/peering/prometheus_exporter.go:46 +0x311 fp=0xc002c3afe0 sp=0xc002c3ad48 pc=0xbedd91
runtime.goexit()
    /usr/local/go/src/runtime/asm_amd64.s:1581 +0x1 fp=0xc002c3afe8 sp=0xc002c3afe0 pc=0x467cc1
created by github.com/migalabs/armiarma/src/peering.(*PeeringService).ServeMetrics
    /home/cortze/cortze/armiarma/src/peering/prometheus_exporter.go:26 +0x28f

It reports that two goroutines are simultaneously reading and writing on the same map, crashing the tool.

Proposed solution:

• [x] Add atomic iterations to any single map iteration