What does this PR do?

Locks trace if sampling priority is set on context propagation.

It also fixes the precedence of global trace rate over trace sampling rules by making sure trace sampling rules are checked before than the global trace rate, as explained here:

// traceRulesSampler allows a user-defined list of rules to apply to traces.
// These rules can match based on the span's Service, Name or both.
// When making a sampling decision, the rules are checked in order until
// a match is found.
// If a match is found, the rate from that rule is used.
// If no match is found, and the DD_TRACE_SAMPLE_RATE environment variable
// was set to a valid rate, that value is used.
// Otherwise, the rules sampler didn't apply to the span, and the decision
// is passed to the priority sampler.

Note for future travelers: our internal logic for sampling works on the basis that a span can be re-sampled during the lifespan of a local span, as we allow sampling rules over tags and we support a bunch of cases where it's also allowed. It's not allowed to re-sample a propagated sampling priority in a downstream service (it'd render head-based sampling not reliable). Thus making the need to lock the trace from the root span if it already has a sampling decision in it.

Motivation

Fixes DataDog/system-tests#5014.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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