Span name with stack trace

[0daryo]

Is your feature request related to a problem? Please describe.
When I write span code, I often write code like below.

StartSpan(ctx, "function name")

using StartSpan function
e.g. https://github.com/googleapis/google-cloud-go/blob/e43c095c94e44a95c618861f9da8f2469b53be16/bigquery/dataset.go#L114

This is a bit tiring and easy to mistake.
So I propose adding helper method that starts span with function name.

Describe the solution you'd like
Adding helper method that calls StartSpan function with function name extracted by stack frame.