contrib/dimfeld/httptreemux.v5: default resource namer incorrectly replaces path variables

[MNThomson]

What does this PR do?

This PR fixes a case where the path variable replacement fails under certain conditions. The current implementation "end of path replacement" isn't actually bound to the end of the path and can do a replacement anywhere in the string. E.x. a parameters value is he and the url is /hey/he it will be replaced as /:?y/he (which obviously isn't intended behaviour).

dd-trace-go/contrib/dimfeld/httptreemux.v5/httptreemux.go

Lines 145 to 148 in 054d19d

	// replace parameter at end of the path, i.e. "../:param"
	oldP = "/" + v
	newP = "/:" + k
	route = strings.Replace(route, oldP, newP, 1)

Motivation

Fixes #2812

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.

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