Add changelogs for PR vitessio#15911 and vitessio#15919

Signed-off-by: Tim Vaillancourt <tim@timvaillancourt.com>

changelog/20.0/20.0.0/summary.md

@@ -36,6 +36,9 @@
  - [New minimum for `--buffer_min_time_between_failovers`](#buffer_min_time_between_failovers-flag)
  - [New `track-udfs` vtgate flag](#vtgate-track-udfs-flag)
  - [Help text fix for `--lock-timeout`](#documentation-lock-timeout)
+ - [New `--querylog-sample-rate` flag](#querylog-sample-rate-flag)
+ - [New `--tablet-filter-tags` flag](#tablet-filter-tags-flag)
+
 - **[Minor Changes](#minor-changes)**
  - **[New Stats](#new-stats)**
  - [VTTablet Query Cache Hits and Misses](#vttablet-query-cache-hits-and-misses)
@@ -357,6 +360,14 @@ The new `--track-udfs` flag enables VTGate to track user defined functions for b
 
 The help text for the flag `--lock-timeout` was incorrect. We were documenting it as a flag that controlled the duration for which the shard lock was acquired. It is actually the maximum duration for which we wait while attempting to acquire a lock from the topology server.
 
+#### <a id="querylog-sample-rate-flag"/>New `--querylog-sample-rate` flag
+
+The new flag `--querylog-sample-rate float` adds support for sampling queries based on a float value between 0.0 _(no logging)_ and 1.0 _(all queries logged)_. If configured, this filtering is applied after the existing `--querylog-filter-tag` filter.
+
+#### <a id="tablet-filter-tags-flag"/>New `--tablet-filter-tags` flag
+
+The new flag `--tablet-filter-tags StringMap` adds support to VTGate for filtering tablets by tablet tag key/values, specified as comma-separated list of key:values. The tags of a tablet are defined by the VTTablet flag `--init_tags`, which is also defined as a comma-separated list of key:values.
+
 ## <a id="minor-changes"/>Minor Changes
 
 ### <a id="new-stats"/>New Stats