stackdriver: parse trace context without regex

[djc]

Changes

Simplify parsing trace context to avoid the complexity of regular expressions (and the added dependency).

Follow-up for #25.

Merge requirement checklist

• [x] CONTRIBUTING guidelines followed
• [x] Unit tests added/updated (if applicable)
• [x] Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• [x] Changes in public API reviewed (if applicable)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 49.9%. Comparing base (4ef9bcd) to head (0bd8beb). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #47 +/- ## ======================================= - Coverage 50.3% 49.9% -0.5% ======================================= Files 32 32 Lines 4386 4358 -28 ======================================= - Hits 2209 2177 -32 - Misses 2177 2181 +4 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[djc]

Ping? I'd like to move forward with getting an opentelemetry-stackdriver release out.

[djc]

Thanks for the review!

The code that is changed here was added in #25 (from January), which was merged after the latest release (0.18.0, from November). So from a downstream perspective, there is no change in this PR.