enhancement(observability): Add configuration for buffer utilization EWMA alpha

[bruceg]

Summary

Adds a buffer_utilization_ewma_alpha configuration option to the global options, allowing users to control the alpha value for the exponentially weighted moving average (EWMA) used in source and transform buffer utilization metrics.

Follow-up to #24453

Vector configuration

How did you test this PR?

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

Notes

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