feat: tracing support for individual entries in bulk subscriptions

[matheusandre1]

Description

Clarifies how tracing works for bulk subscriptions and shows how to get the trace context of each event.

When Dapr delivers messages to a bulk subscriber (@BulkSubscribe over HTTP or onBulkTopicEvent over gRPC), the whole batch arrives in a single call from the sidecar. That call has one trace context, which is the one in the HTTP headers / gRPC metadata. This is expected: it represents the bulk delivery, not the individual messages.

Each message's own W3C trace context is still available per entry, inside its CloudEvent:

• HTTP: CloudEvent#getTraceParent() / CloudEvent#getTraceState() on each entry.getEvent()
• gRPC: the traceparent / tracestate keys in entry.getCloudEvent().getExtensions()

Issue reference

We strive to have all PR being opened based on an issue, where the problem or feature have been discussed prior to implementation.

Closes: #1753

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Extended the documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.11%. Comparing base (c6ad42f) to head (5a86145).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1822   +/-   ##
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  Coverage     79.11%   79.11%           
  Complexity     2604     2604           
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  Files           264      264           
  Lines          7785     7785           
  Branches        820      820           
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  Hits           6159     6159           
  Misses         1266     1266           
  Partials        360      360           

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