Remove STDOUT output in Newrelic exporter (open-telemetry#404)

Remove Note about downgrading Guzzle (open-telemetry#340)

opentelemetry-php now ships with ` "guzzlehttp/guzzle": "^7.1.0"`. See: https://github.com/open-telemetry/opentelemetry-php/blob/main/composer.json#L9

This is out of the box compatible with Laravel 8. Therefore this change removes the note about needing to downgrade Guzzle.

remove global span (open-telemetry#171)

* remove global span creation when Tracer is initialized and fix batch exporter not exporting in batches but one by one

* use the monotonic clock instead of realtime clock to compute time elapsed - realtime clocks  can move backwards

* add the NoopSpan. It will be used as the root Span

* Use NoopSpan for new traces

* Generate a new context if the root span is noop

* Remove comments

* Inline variable

* Must set parent as active span so that span ID is propagated

* Revert "Must set parent as active span so that span ID is propagated" as context can't be used as span

This reverts commit 985fa5e.

* Load NoopSpan when required

* Fixed regression: creating a Tracer with an existing context is now supported again.

* Add variable to improve readability

* Rename variable for clarity.

* fix failing psalm checks and add test for root NoopSpan

Co-authored-by: Ben Magee <ben@bmagee.com>