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.