In taskcluster-lib-api all handlers get access to the context of an api via this inside the handler itself.
Most values are passed through directly to the handler when used but some special Taskcluster libraries
are updated to an instance that is per-request. This is useful for things like having a requestId added
automatically to all log messages made from within a handler and passing along traceId to a further call
made with a Taskcluster client. For Taskcluster library authors, you can make an instance of your library
support this by having a taskclusterPerRequestInstance function on your object that takes requestId
and traceId as named arguments and returns an instance of itself that will live for the lifespan of the
request and be used within. Both taskcluster-lib-monitor and taskcluster-client support this and
are good examples of how this can be used.
traceId is passed around as a header x-taskcluster-trace-id between services. If an external request
sets this header, it will be used. To avoid misuse of this, we recommend running Taskcluster behind
a reverse proxy that will set this header itself and then using k8s-dns serviceDiscoveryScheme for
your services.