Disallow `new Span()`

[dyladan]

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Currently it is possible to call new Span(). We should make the constructor private to disallow this.

Current test usages:

• 1 use of new span in contrib tests: https://github.com/open-telemetry/opentelemetry-js-contrib/blob/main/plugins/node/opentelemetry-instrumentation-dns/test/functionals/utils.test.ts#L150
• ? uses of new Span in JS tests
• honeycomb distro is known to use Span in tests
• dd-trace may also use new Span in tests? @dyladan will look into this

Possible additional goals:

• do not use a class for span (improve minification)
• create test utility library