Commits on Oct 24, 2023

1. Add mostrecent aggregation to Gauge …

   In the multiprocess mode, the process that exposes the metrics needs to
   aggregate the samples from other processes. Gauge metric allows users to
   choose the aggregation mode. This implements 'mostrecent' (and
   'livemostrecent') mode where the last observed value is exposed.
   
   In order to support this, the file format is expanded to store the
   timestamps in addition to the values. The stored timestamps are read by
   the reader process and it's used to find the latest value.
   
   Closes prometheus#847
   
   Consideration on the atomicity:
   
   Previously, mmap_dict.py had a comment saying "We assume that reading
   from an 8 byte aligned value is atomic". With this change, the value
   write becomes a 16 bytes 8-byte aligned write. The code author tried to
   find a basis on the original assumption, but couldn't find any.
   According to write(2), **if a file descriptor is shared**, the write
   becomes atomic. However, we do not share the file descriptors in the
   current architecture.
   
   Considering that Ruby implementation also does the same and hadn't seen
   an issue with it, this write atomicity problem might be practically not
   an issue.
   
   See also:
   
   * prometheus/client_ruby#172
   
     The approach and naming are taken from client_ruby.
   
   * https://github.com/prometheus/client_golang/blob/v1.17.0/prometheus/metric.go#L149-L161
   
     client_golang has an API for setting timestamp already. It explains
     the use case for the timestamp beyond the client-local aggregation. In
     order to support the same use case in Python, further changes are
     needed.
   
   Signed-off-by: Masaya Suzuki <draftcode@gmail.com>

   

   draftcode committed Oct 24, 2023
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