Check that ActiveMerchant::Billing::Gateway.implementations is defined before trying to call it for instrumentation
…t from the browser)
Fixes issue in 1.9.2 where top level constants are detected by MetricParser.const_defined? and therefore erroneously initialized as metric parsing classes.
Rather than a boolean value in a cookie used to signal capturing a trace - a token identifier is passed into the agent from the beacon via the browser cookie. The agent captures a trace and echos the token identifier back to the RUM footer (and beacon). This allows the beacon to know which requests return.
This reverts commit 595039a.