What does this PR do?

Port of #4734 to main.

ddtrace/tracer/stats.go — 3 changes

1. tracerStatSpan gets a version field

type tracerStatSpan struct {
    statSpan *stats.StatSpan
    origin   string
    version  string  // per-span version tag; "" means use global aggKey version
}

The struct that carries a finished span to the concentrator's goroutine now also carries whatever version meta tag was on the span.

2. newTracerStatSpan captures ext.Version from the span meta

return &tracerStatSpan{
    statSpan: statSpan,
    origin:   s.meta[keyOrigin],
    version:  s.meta[ext.Version],  // ← new
}, true

Called on the hot path when a span finishes. Reads s.meta["version"] — the same tag synthtracer sets via tracer.Tag("version", currentVersion()).

3. add() overrides the payload aggregation key version per-span

func (c *concentrator) add(s *tracerStatSpan) {
    aggKey := c.aggregationKey          // value copy — c.aggregationKey unchanged
    if s.version != "" {
        aggKey.Version = s.version      // per-span version wins
    }
    c.spanConcentrator.AddSpan(s.statSpan, aggKey, "", nil, s.origin)
}

c.aggregationKey is built once at startup with Version = c.internalConfig.Version() (empty for synthtracer). Before this fix, every span used that frozen empty version → N/A. Now each span can carry its own version into a separate stats bucket, matching what the agent-side concentrator does when computing server-side stats.

ddtrace/tracer/stats_test.go — TestPerSpanVersionInStats

Three sub-tests that pin the behaviour:

Motivation

Ensure that stats are properly tagged with the version, both global and per-span.

Root cause:

• PR feat(tracer): periodically poll agent /info endpoint for dynamic capability updates #4451 (commit 6e49f81) introduced periodic /info polling every 5s in v2.8.0.
• DropP0s + Stats are dynamic fields refreshed by that poll.
• Once the sidecar agent's client_drop_p0s: true is detected (after startup), the concentrator activates.
• The concentrator's aggregationKey.Version was set at construction from c.internalConfig.Version() = "" (no global version).
• All client-side stats are emitted with version="" → N/A.
• The agent-side concentrator reads version from each span's meta["version"] via pt.AppVersion, so it always picked up the rotating tag. Client-side does not.

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 make lint locally.
• New code doesn't break existing tests. You can check this by running make test locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• All generated files are up to date. You can check this by running make generate locally.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild. Make sure all nested modules are up to date by running make fix-modules locally.

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