ddtrace/tracer: update origin substitution in composeTracestate

[lievan]

What does this PR do?

This PR updates the substitution of invalid characters in origin-value for trace state. Previously, the invalid character = was being substituted with _, which was incorrect and caused a case in test_headers_tracestate_dd.py to fail in the parametric tests. This PR updates = to be substituted with ~ before the rest of the invalid characters are replaced with _, which causes the parametric test case to pass.

Motivation

Update the substitution of invalid characters in origin to match the correct behavior W3C trace context propagation and fix a failing test case in parametric tests.

Describe how to test/QA your changes

Checkout to shevchenko/w3c-golang in the system-tests repo, update the dd-trace-go module in apps/golang to this PR, and run trace_headers tests

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.

[dianashevchenko]

If ~ will be unescaped into = during extraction, we must first replace all the invalid characters including tilde with _ and then replace = with ~

[pr-commenter]

Benchmarks

Comparing candidate commit f8e4695 in PR branch evan.li/update-origin-sub with baseline commit 7f6f95d in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 6 cases.