* Fix some tracing issues.
- Add endpoints for `startUpdate` and `postEngineEventsBatch` so that
spans for these invocations have proper names
- Inject a tracing span when walking a plan so that resource operations
are properly parented
- When handling gRPC calls, inject a tracing span into the call's
metadata if no span is already present so that resource monitor and
engine spans are properly parented
- Do not trace client gRPC invocations of the empty method so that these
calls (which are used to determine server availability) do not muddy
the trace. Note that I tried parenting these spans appropriately, but
doing so broke the trace entirely.
With these changes, the only unparented span in a typical Pulumi
invocation is a single call to `getUser`. This span is unparented
because that call does not have a context available. Plumbing a context
into that particular call is surprisingly tricky, as it is often called
by other context-less functions.
* Make tracing support more flexible.
- Add support for writing trace data to a local file using Appdash
- Add support for viewing Appdash traces via the CLI
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