refactor(config): add cross-product gate to prevent conflicting programmatic overrides (#4640)

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### What does this PR do?
Adds a cross-product gate to internal/config that prevents different
products (tracer, profiler, etc.) from setting conflicting values for
the same configuration field via programmatic APIs. When a conflict is
attempted, a new telemetry metric `config.product_conflict` is emitted.

Every `Set*` method now accepts an optional `...Product` parameter. When
a product passes its identity (e.g. `ProductTracer`), the gate records
that claim. If a different product later tries to set the same field,
the call is rejected (first-in-wins) and a warning is logged.

Env vars, defaults, and non-programmatic origins bypass the gate
entirely.
Tests and integrations omit the product parameter and are unaffected.

### Motivation

Previously, each product (tracer, profiler, etc) loaded configuration
independently from all sources and stored its own local copy.
Programmatic APIs like `tracer.WithService` or `profiler.WithEnv` would
overwrite only that product's local copy. As we consolidate into a
single shared Config instance in internal/config, programmatic APIs from
different products now write to the same fields. The gate ensures these
writes don't silently conflict: the first product to claim a field via
programmatic API owns it, and attempts from other products are rejected
with a warning.

### Reviewer's Checklist
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- [ ] Changed code has unit tests for its functionality at or near 100%
coverage.
- [ ] [System-Tests](https://github.com/DataDog/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.

Unsure? Have a question? Request a review!

Author: mtoffl01

ddtrace/tracer/option.go

@@ -376,27 +376,27 @@ func newConfig(opts ...StartOption) (*config, error) {
 	if c.internalConfig.Env() == "" {
 		if v, ok := globalTags["env"]; ok {
 			if e, ok := v.(string); ok {
-				c.internalConfig.SetEnv(e, c.globalTags.Origin())
+				c.internalConfig.SetEnv(e, c.globalTags.Origin(), internalconfig.ProductTracer)
 			}
 		}
 	}
 	if c.internalConfig.Version() == "" {
 		if v, ok := globalTags["version"]; ok {
 			if ver, ok := v.(string); ok {
-				c.internalConfig.SetVersion(ver, c.globalTags.Origin())
+				c.internalConfig.SetVersion(ver, c.globalTags.Origin(), internalconfig.ProductTracer)
 			}
 		}
 	}
 	if c.internalConfig.ServiceName() == "" {
 		if v, ok := globalTags["service"]; ok {
 			if s, ok := v.(string); ok {
-				c.internalConfig.SetServiceName(s, c.globalTags.Origin())
+				c.internalConfig.SetServiceName(s, c.globalTags.Origin(), internalconfig.ProductTracer)
 				globalconfig.SetServiceName(s)
 			}
 		} else {
 			// There is not an explicit service set, default to binary name.
 			// In this case, don't set a global service name so the contribs continue using their defaults.
-			c.internalConfig.SetServiceName(filepath.Base(os.Args[0]), internalconfig.OriginDefault)
+			c.internalConfig.SetServiceName(filepath.Base(os.Args[0]), internalconfig.OriginDefault, internalconfig.ProductTracer)
 		}
 	} else {
 		globalconfig.SetServiceName(c.internalConfig.ServiceName())
@@ -960,7 +960,7 @@ func WithPropagator(p Propagator) StartOption {
 // WithService sets the default service name for the program.
 func WithService(name string) StartOption {
 	return func(c *config) {
-		c.internalConfig.SetServiceName(name, internalconfig.OriginCode)
+		c.internalConfig.SetServiceName(name, internalconfig.OriginCode, internalconfig.ProductTracer)
 		globalconfig.SetServiceName(name)
 	}
 }
@@ -1031,7 +1031,7 @@ func WithAgentTimeout(timeout int) StartOption {
 // The default value is the environment variable DD_ENV, if it is set.
 func WithEnv(env string) StartOption {
 	return func(c *config) {
-		c.internalConfig.SetEnv(env, telemetry.OriginCode)
+		c.internalConfig.SetEnv(env, telemetry.OriginCode, internalconfig.ProductTracer)
 	}
 }
 
@@ -1182,7 +1182,7 @@ func WithSamplingRules(rules []SamplingRule) StartOption {
 // span service name and config service name match. Do NOT use with WithUniversalVersion.
 func WithServiceVersion(version string) StartOption {
 	return func(cfg *config) {
-		cfg.internalConfig.SetVersion(version, telemetry.OriginCode)
+		cfg.internalConfig.SetVersion(version, telemetry.OriginCode, internalconfig.ProductTracer)
 		cfg.universalVersion = false
 	}
 }
@@ -1192,7 +1192,7 @@ func WithServiceVersion(version string) StartOption {
 // See: WithService, WithServiceVersion. Do NOT use with WithServiceVersion.
 func WithUniversalVersion(version string) StartOption {
 	return func(c *config) {
-		c.internalConfig.SetVersion(version, telemetry.OriginCode)
+		c.internalConfig.SetVersion(version, telemetry.OriginCode, internalconfig.ProductTracer)
 		c.universalVersion = true
 	}
 }

internal/config/README.md

@@ -9,13 +9,29 @@ When migrating a configuration value from another package (e.g. `ddtrace/tracer`
 - **Define the field on `Config`**: add a private field on `internal/config.Config`.
 - **Initialize it in `loadConfig()`**: read from the config provider, which iterates over the following sources, in order, returning the default if no valid value found: local declarative config file, OTEL env vars, env vars, managed declarative config file
 - **Expose an accessor**: add a getter (and a setter if the value is updated at runtime).
-- **Report telemetry in setters**: setters should call `reportTelemetry(...)` with the correct origin.
+- **Report telemetry in setters**: setters should call `configtelemetry.Report(...)` with the correct origin.
+- **Add the cross-product gate**: every setter must call `c.checkProductConflict(...)` as its first action after acquiring the lock (see below).
 - **Update callers**: replace reads/writes on local "config" structs with calls to the singleton (`internal/config.Get()`).
 - **Delete old state**: remove the migrated field from any legacy config structs once no longer referenced.
 - **Update tests**: tests should call the singleton setter/getter (or set env vars) rather than mutating legacy fields.
 
 Sample migration PR: https://github.com/DataDog/dd-trace-go/pull/4214
 
+## Cross-product gate
+
+Every `Set*` method accepts an optional trailing `...Product` parameter. When a product (tracer, profiler, etc.) sets a field via its programmatic API, it passes its `Product` identity:
+
+```go
+c.internalConfig.SetServiceName(name, internalconfig.OriginCode, internalconfig.ProductTracer)
+```
+
+The gate enforces **first-in-wins**: if a different product already claimed the field via programmatic API, the call is silently rejected and a warning is logged. This prevents conflicting overrides like `tracer.WithService("A")` + `profiler.WithService("B")`.
+
+Key rules:
+- **Env vars, defaults, and RC always pass through** — the gate only activates for `OriginCode`.
+- **Tests and integrations omit the product** — they call `SetServiceName(name, origin)` without a product, bypassing the gate entirely.
+- **Same product can call a setter multiple times** — repeated calls from the same product just update the value.
+
 ## Hot paths & performance guidelines
 
 Some configuration accessors may be called in hot paths (e.g., span start/finish, partial flush logic).