[exporter/prometheusremotewrite] fix snappy double alloc

[ben-childs-docusign]

Description:

Snappy Encode function is not using the buffer passed in by prometheus remote write as it is too small. Removing this unused buffer allocation and just passing nil. Snappy will allocate the required buffers.

Link to tracking Issue:
Fixes #34273

Testing:
Ran existing unit tests which cover this code

Ran this in our integration environment and verified reduced allocations.
Before:

After:

Documentation:
Updated changelog

[rapphil]

There are a few approval steps that are failing, can you take a look to make sure that they are not related to this change or require merging from mainline?

Otherwise this change looks good to me.

[ben-childs-docusign]

@mwear , @rapphil thanks for the review - whats the next step to get this merged? I checked the latest build failure and it seems to be a transformprocessor unit test which isn't impacted by this change afaict.

[rapphil]

@ben-childs-docusign , It is possible that it is a flaky test. can you take a look into other PRs that were created recently and check if the tests are failing there as well? Is your branch updated with regards to main? As next steps, I think @mwear can add the ready to merge label and then some maintainer will pick it for merging.