Modify KvToString() Method in Instrument.h to Allow Valid Label Names in Records For Prometheus

[erichsueh3]

This PR is in reference to issue #297 and changes the KvToString() method in Instrument.h. Additionally, there are changes to the print_value() method because KvToString() calls this method for translating a label’s value. Finally, the boolean jsonTypes parameter in print_value() is left unused; this PR does not remove it, though it does not see use anywhere else and has been left in as a precautionary measure.

cc @alolita, @reyang, @pyohannes

[codecov]

Codecov Report

Merging #298 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #298      +/-   ##
==========================================
- Coverage   94.60%   94.59%   -0.01%     
==========================================
  Files         146      146              
  Lines        6633     6629       -4     
==========================================
- Hits         6275     6271       -4     
  Misses        358      358              

Impacted Files	Coverage Δ
sdk/include/opentelemetry/sdk/metrics/instrument.h	93.84% <100.00%> (-0.36%)	⬇️
sdk/test/metrics/metric_instrument_test.cc	100.00% <100.00%> (ø)

[erichsueh3]

The Bazel on MacOS test seems to be failing due to a missing dependency in zpages; not sure if this is related to this PR or not.

[erichsueh3]

@ankit-bhargava @Brandon-Kimberly Do you two have any insight on why a double quote character is needed in a label name? I'm not sure if I'm missing something in the specification or if there's another reason for this

[Brandon-Kimberly]

@ankit-bhargava @Brandon-Kimberly Do you two have any insight on why a double quote character is needed in a label name? I'm not sure if I'm missing something in the specification or if there's another reason for this

Ankit implemented the function but as far as I am aware there is no reason for it to be there. I imagine it was just added because it makes it look nicer when exported to stdout. But again, I'm not sure if there is a reason for it.