Clarify the data format for httptracker's spans method

[githubsands]

It is still misunderstood whether to use a list of map[string]string or a list of strings to store a trace's spans that allow optimal conditions to communicate with WRP. Spans() []string method must be updated when this issue has come to fruition.

type Tracker interface {
          ....
	// Spans returns a list of string-encoded Money spans that have been created under this tracker
	Spans() []string
}