[chore] Add sharded batcher wrapper for unpartitioned batching

[iblancasa]

Description

Adds an internal shardedBatcher wrapper for the unpartitioned exporterhelper batch path. It keeps behavior unchanged by constructing the wrapper with exactly one shard. The shard itself is the existing partitionBatcher, so current batching semantics for min_size, flush_timeout, shutdown draining, and request completion remain delegated to the existing implementation.

This is a first step to allow unpartitioned batching to use multiple independent batch streams (follow up PR).

Link to tracking issue

Updates #12473

Testing

• Local manual testing
• make test in exporterhelper

Authorship

• I, a human, wrote this pull request description myself.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.05%. Comparing base (9ee240d) to head (7911c3f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15460   +/-   ##
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  Coverage   91.04%   91.05%           
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  Files         728      729    +1     
  Lines       48467    48488   +21     
=======================================
+ Hits        44129    44150   +21     
  Misses       3011     3011           
  Partials     1327     1327           

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[mwear]

Mostly LGTM. I understand that the goal is to keep behavior unchanged by routing through a single shard. But the round-robin multi-shard routing is already implemented and reachable, and every test uses one shard, so that distribution path never actually runs. What do you think about a shardCount > 1 test that checks requests get spread across shards and that all shards flush and drain correctly?

[iblancasa]

@mwear I was planning to add this in follow up PRs but, at the same time, understand that it can be interesting to see this now. Added.

[mwear]

Thanks @iblancasa.

[dmitryax]

@iblancasa can you please submit a follow-up PR on top of this in draft state? I'd like to see how you plan this to be implemented before merging this.

[iblancasa]

@iblancasa can you please submit a follow-up PR on top of this in draft state? I'd like to see how you plan this to be implemented before merging this.

Done #15534