Update Docs with right contract of MeterFilter

[lenin-jaganathan]

Until 1.13 the meter filters are executed every time the builder is called. This was changed in 1.13.x to provide performance benefits while using builders. This has broken the use-cases where MeterFilters were used to add dynamic content to metrics from a local context/threadlocals etc. Make this change explicit in the docs so that users have right expectation for MeterFilters.

[shakuzen]

Thanks for putting this on the radar explicitly. The intention has been to document this, but we wanted to make sure we got enough feedback about the change and could make the updated documentation as helpful based on common scenarios in which MeterFilter was used in ways contrary to what the change expects. I think it may be helpful to update something related to this in 1.12.x even if it may be a bit different wording compared to 1.13.x where the concrete change was made.