include action time and value size in debug logs

Implements new features per the issue description:
https://git.drupalcode.org/project/memcache/-/merge_requests/13/diffs

Includes new documentation:

+You can also filter to only show requests that take longer than a specific number
+of seconds. For example, the following would only log memcache actions that take
+longer than 0.5 seconds:
+ $conf['memcache_debug_elapsed_minimum'] = 0.5;
+
+To include '!size' (the estimated size in bytes of the object being written to
+or read from memcache) enable the following option:
+ $conf['memcache_debug_size'] = TRUE;
+
+In order to calculate the size of objects, they are first serialized then strlen
+is used to determine total bytes. While this may not always be precise, it is a
+close enough estimation to be a useful metric in debug logging. Note that there
+is added CPU overhead when enabling this option, due to all the serialization
+required.
+
+If enabled, you can also filter what is logged, so for example only objects over
+1024 bytes in size:
+ $conf['memcache_debug_size_minimum'] = 1024;

I created some very large nodes, and then loaded them with this enabled to get the following logs:

1680863171|429.34|getMulti|cache_field|cache_field-cache-_multi181-field%253Anode%253A6|1048151|1
1680863171|430.37|getMulti|cache_field|cache_field-cache-_multi182-field%253Anode%253A6|1045479|1
1680863172|931.11|getMulti|cache_field|cache_field-field%3Anode%3A6|191789014|1
1680863200|0.27|get|cache_bootstrap|cache_bootstrap-system_list|6348|1

In my test configuration I configured the debug log to only include objects over 5120 bytes and that take longer than 0.25 milliseconds to process:

$conf['memcache_debug_size'] = TRUE;
$conf['memcache_debug_size_minimum'] = 5120;
$conf['memcache_debug_elapsed_minimum'] = 0.25;

The first two rows are part of a getMulti request for a large object was split into 182 pieces. You can see that each request is taking ~430 milliseconds, and that the objects have been split into 1,045,479 byte pieces The last line is a slow (0.27 millisecond) request for cache_bootstrap, which is 6,348 bytes in size.