The num_counters argument to llvm.instrprof.increment is the max counter_id + 1, or in
other words, the number of counter value references injected into the MIR (plus 1 for the
reserved ZERO counter, which uses counter ID 0 when included in an expression). Injected
counters have a counter ID from 1..num_counters-1.
The query provider for CoverageInfo, requested by codegen_coverage() (to inject each
counter) and FunctionCoverage::new() (to extract the coverage map metadata from the MIR).