Add support for HTTP templated paths

[andrewslotin]

This PR adds support for forwarding the path template used to match an HTTP request to the handler. The template string can either be specified during the instrumentation by providing it as an argument to instana.TracingHandlerFunc() or added as a custom tag http.path_tpl to the incoming request span.

Providing path template during instrumentation

// Provide the template string as an argument to instana.TracingHandlerFunc()
instrumentedHandler := instana.TracingHandlerFunc(sensor, "/articles/{category}/{id:[0-9]+}", Handler)
// Register your instrumented handler
r.HandleFunc("/articles/{category}/{id:[0-9]+}", instrumentedHandler)

Please note that the agent will attempt to detect whether a request has been routed via pattern matching by comparing it to (*http.Request).Path and only populate the http.path_tpl if these values differ.

Adding a path template to the request span

func Handler(w http.ResponseWriter, req *http.Request) {
	// Extract the request span from context and add an `http.path_tpl` tag to it
	if parent, ok := instana.SpanFromContext(req.Context()); ok {
		parent.SetTag("http.path_tpl", "/articles/{category}/{id:[0-9]+}")
	}

	// ...
}