1. A computer system for tracking a transaction flow in a distributed computing environment, comprising: one or more processors, one or more computer-readable memories and one or more computer-readable, tangible storage devices; program instructions, stored on at least one of the one or more storage devices for execution by at least one of the one or more processors via at least one of the one or more memories, responsive to transactional middleware for a particular domain processing a particular transaction instance at a particular processing point specified in transaction telemetry standards as a tracking point along a flow of the particular transaction, to access, by at least one facility of the transactional middleware, correlation data for the particular transaction instance in the particular domain and extracting, by the at least one facility, application level context information about the particular transaction instance from the particular domain, wherein the transaction telemetry standards specify a same set of standards for application by each separate transactional middleware instance of a plurality of middleware instances of a composite application through each of the different types of facilities of each separate middleware instance; program instructions, stored on at least one of the one or more storage devices for execution by at least one of the one or more processors via at least one of the one or more memories, to create, by the at least one facility of the transactional middleware, a tracking event comprising the correlation data and the application level context information formatted into an event structure defined by the transaction tracking standards for a type of processing point of the tracking point; and program instructions, stored on at least one of the one or more storage devices for execution by at least one of the one or more processors via at least one of the one or more memories, to surface the tracking event, by the at least one facility of the transactional middleware in compliance with the transaction telemetry standards, for access to the tracking event by applications external to the particular domain.