This PR follows up on #14900 and proposes active span restoration when passing a custom scope to startSpanManual. This is unfortunately a bit trickier than for startSpan because there are two ways how users can end a span:

• by using span.end()
• by using the finish function passed to the startSpanManual callback

With the changes in this PR, this two ways of ending a span would diverge in behaviour:

startSpanManual({ scope: customScope }, (span, finish)  => {
  // ends the span and restores the previous active span on the scope
  finish();
})

startSpanManual({ scope: customScope }, span  => {
  // ends the span but DOES NOT restore the span on the scope
  span.end();
})

The alternative to the proposed change would be to monkey-patch span.end() to always restore the active span on custom scopes. Can also change the PR to do this if reviewers prefer it.