feat: add PHP 8 compatibility (census-instrumentation#270)

fix: PHP 7.4 Compatibility Fix (census-instrumentation#259)

- fixes `Deprecated: Array and string offset access syntax with curly braces is deprecated in...`

Prepare 0.5.2 (census-instrumentation#203)

Prepare 0.5.1 (census-instrumentation#193)

Extract exporters (census-instrumentation#175)

* Remove Stackdriver/Zipkin exporters. Update README

* Update links. Remove google/cloud-trace dev dep.

* Add suggestions for the other exporters

* Fix url to api docs

* Put the mock_error_log back

* guzzle 5 does not have psr7 as a dependency

* Fix GitHub org link for the exporters

Version bump v0.4.3 (census-instrumentation#169)

version bump v0.4.2 (census-instrumentation#166)

ExtensionTracer should set traceId on spans (census-instrumentation#157)

* Failing test for extension not preserving traceId

* ExtensionTracer correctly sets traceId on returned spans

Prepare PECL 0.2.0 (census-instrumentation#155)

* Prepare PECL 0.2.0

* Fix time

Simplify quickstart (census-instrumentation#142)

* Simplify silex example for use as quickstart

* Update silex README with updates