ddtrace/mocktracer: fix SetUser & WithUserMetadata

[eliottness]

What does this PR do?

This PR fixes a panic happening when using tracer.SetUser with the option WithUserMetadata on a instance of a mocktracer.

Stacktrace:

panic: assignment to entry in nil map [recovered]
	panic: assignment to entry in nil map

goroutine 5 [running]:
testing.tRunner.func1.2({0x10487e640, 0x104965140})
	/opt/homebrew/Cellar/go/1.22.3/libexec/src/testing/testing.go:1631 +0x1c4
testing.tRunner.func1()
	/opt/homebrew/Cellar/go/1.22.3/libexec/src/testing/testing.go:1634 +0x33c
panic({0x10487e640?, 0x104965140?})
	/opt/homebrew/Cellar/go/1.22.3/libexec/src/runtime/panic.go:770 +0x124
github.com/DataDog/dd-source/domains/aaa/shared/libs/go/authctxhttp.Authenticate.func1.1.WithUserMetadata.4(0x1400008db18?)
	/Users/kaitlyn.paglia/go/pkg/mod/gopkg.in/!data!dog/dd-trace-go.v1@v1.66.0-rc.1/ddtrace/tracer/option.go:1326 +0x40
gopkg.in/DataDog/dd-trace-go.v1/ddtrace/mocktracer.(*mockspan).SetUser(0x104de8fb0?, {0x14000354154, 0x4}, {0x1400007c2e0, 0x3, 0x14000358000?})
	/Users/kaitlyn.paglia/go/pkg/mod/gopkg.in/!data!dog/dd-trace-go.v1@v1.66.0-rc.1/ddtrace/mocktracer/mockspan.go:251 +0x70
gopkg.in/DataDog/dd-trace-go.v1/ddtrace/tracer.SetUser({0x104971c98?, 0x14000358000?}, {0x14000354154?, 0x4?}, {0x1400007c2e0?, 0x3?, 0x4?})
	/Users/kaitlyn.paglia/go/pkg/mod/gopkg.in/!data!dog/dd-trace-go.v1@v1.66.0-rc.1/ddtrace/tracer/tracer.go:224 +0x7c
gopkg.in/DataDog/dd-trace-go.v1/appsec.SetUser({0x10496f0c8, 0x1400035d050}, {0x14000354154, 0x4}, {0x1400007c2e0, 0x3, 0x4})
	/Users/kaitlyn.paglia/go/pkg/mod/gopkg.in/!data!dog/dd-trace-go.v1@v1.66.0-rc.1/appsec/appsec.go:58 +0x54
github.com/DataDog/dd-source/domains/aaa/shared/libs/go/authctxhttp.Authenticate.func1.1({0x10496d6c8, 0x14000346040}, 0x14000422120)
	/Users/kaitlyn.paglia/go/src/github.com/DataDog/dd-source/domains/aaa/shared/libs/go/authctxhttp/authctxhttp.go:337 +0xc18
net/http.HandlerFunc.ServeHTTP(0x104967bd8?, {0x10496d6c8?, 0x14000346040?}, 0x1046c8f17?)
	/opt/homebrew/Cellar/go/1.22.3/libexec/src/net/http/server.go:2166 +0x38
github.com/DataDog/dd-source/domains/aaa/shared/libs/go/authctxhttp.TestAppSecBlocking(0x14000111ba0)
	/Users/kaitlyn.paglia/go/src/github.com/DataDog/dd-source/domains/aaa/shared/libs/go/authctxhttp/authctxhttp_test.go:290 +0x3a4
testing.tRunner(0x14000111ba0, 0x104961ec0)
	/opt/homebrew/Cellar/go/1.22.3/libexec/src/testing/testing.go:1689 +0xec
created by testing.(*T).Run in goroutine 1
	/opt/homebrew/Cellar/go/1.22.3/libexec/src/testing/testing.go:1742 +0x318
FAIL	github.com/DataDog/dd-source/domains/aaa/shared/libs/go/authctxhttp	0.642s
FAIL

Motivation

Happened in dd-source on a rapid service testing appsec.SetUser

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.
• 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.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-07-15 08:16:42

Comparing candidate commit 75c34ed in PR branch eliott.bouhana/mockspan-set-user with baseline commit a87c895 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 46 metrics, 1 unstable metrics.