[exporterhelper] Preserve request span context in the persistent queue

[dmitryax]

This change makes it possible to propagate span context through the Collector with enable persistent queue.

Currently, it is behind the exporter.PersistRequestContext feature gate, which can be enabled by adding --feature-gates=exporter.PersistRequestContext to the collector command line. An exporter buffer stored by a previous version of the collector (or by a collector with the feature gate disabled) can be read by a newer collector with the feature enabled. However, the reverse is not supported: a buffer stored by a newer collector with the feature enabled cannot be read by an older collector (or by a collector with the feature gate disabled).

Resolves #11740

Alternative to #13176 that doesn't involve custom encoder but exposes new public module pdata/xpdata/request

The actual change set is pretty small. Most of the code is generated protobuf

[codecov]

Codecov Report

Attention: Patch coverage is 95.26316% with 9 lines in your changes missing coverage. Please review.

Project coverage is 91.53%. Comparing base (1921e21) to head (efcbe18).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
exporter/exporterhelper/logs.go	88.23%	2 Missing ⚠️
exporter/exporterhelper/metrics.go	88.23%	2 Missing ⚠️
exporter/exporterhelper/traces.go	88.23%	2 Missing ⚠️
...xporter/exporterhelper/xexporterhelper/profiles.go	88.23%	2 Missing ⚠️
.../exporterhelper/internal/queue/persistent_queue.go	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13188      +/-   ##
==========================================
+ Coverage   91.27%   91.53%   +0.26%     
==========================================
  Files         514      520       +6     
  Lines       28857    28918      +61     
==========================================
+ Hits        26340    26471     +131     
+ Misses       1998     1929      -69     
+ Partials      519      518       -1     

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[bogdandrutu]

The only downside is that I don't see how to extend this to preserve other parts from the context (configured somehow).

Let's merge and iterate over a possible version to have instead of context a map[string]string? Or maybe add options to Marshal/Unmarshal and accept some helpers that will return "key/value" and accept "key/value" when re-constructing the context?

[bogdandrutu]

Map is pretty expensive, can keep a list of message {key, value} since this is not exposed to the users.