OTLP: Add metrics to track otlp request samples per batch

[ying-jeanne]

What this PR does

This PR intent to address #6935 (comment), we need to use the same limit unit for otel collector and mimir, it should be number of samples per batch and aligned with send_batch_max_size. Before doing so, we need better understanding what are the batch size now from users.

The added bucket is based on our recommendation 8192

Note: there is no fancy way to test native histogram metrics exported today.

Which issue(s) this PR fixes or relates to

Fixes: #8269

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.

[pstibrany]

Unfortunately this will create a lot of series (number of tenants using OTLP * number of distributors * (number of buckets + 2)). It's around 700k series altogether in our production environments. If we used native histograms only, it would be 1/10th of that.

If we go with native histograms, I'd recommend using factor=2, 4 or 16 (buckets would be have boundaries that are exponents of the factor).

[ying-jeanne]

after discussed offline with Peter, I am going to change factor to 2 and max bucket to 100.

[bboreham]

I'd like to suggest we track these numbers for all sources together (remote-write and OTLP), since it is equally interesting for remote-write, and the common case is that each customer picks just one method.

Also it would be good to have the number of exemplars tracked similarly.