fix: clean-up the rate sequence goroutines to avoid leaking

[d1823]

As stated on Ticker.Stop:

Stop does not close the channel, to prevent a concurrent goroutine
reading from the channel from seeing an erroneous "tick".

This sole fact prevents the goroutine from exiting, and in result, causes a leak that is especially problematic when the project is used through an API and not a CLI.

This particular bug is directly affecting the Telegraf's Internet Speed Input Plugin, so this fix is only resolving the part relevant to the plugin.

There are two more occurrences of this pattern:

speedtest-go/speedtest/data_manager.go

Line 103 in 320b5b7

func (dm *DataManager) CallbackDownloadRate(callback func(downRate float64)) *time.Ticker {

speedtest-go/speedtest/data_manager.go

Line 114 in 320b5b7

func (dm *DataManager) CallbackUploadRate(callback func(upRate float64)) *time.Ticker {

Unfortunately, fixing them would probably require an API change, so I'm purposefully leaving that to the maintainers of this project.

[r3inbowari]

@d1823 Thank you very much for your careful debugging! I made a release for telegraf's quickly fixing, and another occurrences(CLI) can be fix in the next.