envcar: refine normalization

[pellared]

Why

From POSIX Standards:

Environment variable names used by the utilities in the Shell and Utilities volume of POSIX.1-2024 consist solely of uppercase letters, digits, and the ('_') from the characters defined in Portable Character Set and do not begin with a digit.

Related to open-telemetry/opentelemetry-specification#4914

What

Change the normalization so that it adds _ in front when the key starts with a digit so that the result is compliant with the POSIX Standards.

goos: linux
goarch: amd64
pkg: go.opentelemetry.io/contrib/propagators/envcar
cpu: 13th Gen Intel(R) Core(TM) i7-13800H
BenchmarkNormalize/#00-20               996370892                1.244 ns/op           0 B/op          0 allocs/op
BenchmarkNormalize/ABC-20               58236038                19.98 ns/op            3 B/op          1 allocs/op
BenchmarkNormalize/abc-20               63575781                19.51 ns/op            3 B/op          1 allocs/op
BenchmarkNormalize/01239-20             49892817                27.72 ns/op            8 B/op          1 allocs/op
BenchmarkNormalize/0abc-20              50815398                25.87 ns/op            5 B/op          1 allocs/op
BenchmarkNormalize/9-20                 81466388                15.07 ns/op            2 B/op          1 allocs/op
BenchmarkNormalize/a_b_c-20             48530017                25.69 ns/op            5 B/op          1 allocs/op
BenchmarkNormalize/hello-world-20       21449659                52.89 ns/op           16 B/op          1 allocs/op
BenchmarkNormalize/foo.bar-20           29744493                33.63 ns/op            8 B/op          1 allocs/op
BenchmarkNormalize/Content-Type-20              22689820                52.31 ns/op           16 B/op          1 allocs/op
BenchmarkNormalize/traceparent-20               24628167                51.74 ns/op           16 B/op          1 allocs/op
BenchmarkNormalize/key_with_spaces-20           20892998                56.24 ns/op           16 B/op          1 allocs/op
BenchmarkNormalize/MiXeD_123!-20                26057121                46.39 ns/op           16 B/op          1 allocs/op
BenchmarkNormalize/🧳-20                        90115291                11.71 ns/op            0 B/op          0 allocs/op
BenchmarkNormalize/Mój_Bagaż-20                 20788340                57.58 ns/op           16 B/op          1 allocs/op

[pellared]

@Joibel PTAL

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.1%. Comparing base (340d326) to head (6da8f54).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8678   +/-   ##
=====================================
  Coverage   82.0%   82.1%           
=====================================
  Files        182     183    +1     
  Lines      13780   13790   +10     
=====================================
+ Hits       11311   11332   +21     
+ Misses      2065    2054   -11     
  Partials     404     404           

Files with missing lines	Coverage Δ
propagators/envcar/carrier.go	100.0% <100.0%> (ø)
propagators/envcar/normalize.go	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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