profile: avoid string(int) conversion

[smasher164]

As per the vet check being introduced in 1.15
(golang/go#32479), conversions of the form
string(int) are flagged by go vet. The suggested fix is to instead use
string(rune).

Additionally, since google/pprof is vendored into the go tree, we avoid
test failures from vet flagging this dependency (see
CL https://go-review.googlesource.com/c/go/+/220977).

[codecov-io]

Codecov Report

Merging #513 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #513   +/-   ##
=======================================
  Coverage   67.06%   67.06%           
=======================================
  Files          37       37           
  Lines        7611     7611           
=======================================
  Hits         5104     5104           
  Misses       2103     2103           
  Partials      404      404

Impacted Files	Coverage Δ
profile/proto.go	83.25% <100%> (ø)	⬆️

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.06%. Comparing base (d9cc468) to head (6cfaca7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #513   +/-   ##
=======================================
  Coverage   67.06%   67.06%           
=======================================
  Files          37       37           
  Lines        7611     7611           
=======================================
  Hits         5104     5104           
  Misses       2103     2103           
  Partials      404      404           

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