Refactoring tracer api

[cappuc]

• Added new SpanBuilder which allows to create and start a span with fluent syntax:

Tracer::newSpan('name')->setSpanKind(SpanKind::KIND_SERVER)->start();

• Removed Tracer::start, Tracer::measure, moved to span builder:

Tracer::newSpan('name')->start();
Tracer::newSpan('name')->measure(callback);

• Removed Tracer::measureAsync (it was like measure with span kind PRODUCER), replaced with:

Tracer::newSpan('name')->setSpanKind(SpanKind::KIND_PRODUCER)->measure(callback);

• Removed Tracer::recordExceptionToSpan, use span methods directly:

$span->recordException($exception)
 ->setStatus(StatusCode::STATUS_ERROR); // optional, recordExceptionToSpan was setting the status to error

• Removed Tracer::setRootSpan($span), it was only used to share traceId with log context. This has been replaced with Tracer::updateLogContext()