Count how many shards were hit by routes when using vexplain

[frouioui]

Description

With this PR we will now track another metric for vexplain trace: the amount of shards hit by each route. This metric will enable us to know if a plan hits more or less shards for the same result, which is an useful indicator of performance.

After running the 30x local examples here is what we get:

mysql> vexplain trace select * from corder\G
*************************** 1. row ***************************
Trace: {
        "OperatorType": "Route",
        "Variant": "Scatter",
        "Keyspace": {
                "Name": "customer",
                "Sharded": true
        },
        "NoOfCalls": 1,
        "AvgNumberOfRows": 5,
        "MedianNumberOfRows": 5,
        "ShardsQueried": 2,
        "FieldQuery": "select order_id, customer_id, sku, price from corder where 1 != 1",
        "Query": "select order_id, customer_id, sku, price from corder",
        "Table": "corder"
}
1 row in set (0.00 sec)

mysql> 

Related Issue(s)

• Part of Benchmark to Measure Impact of Planner Changes vt#21

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: Patch coverage is 97.05882% with 1 line in your changes missing coverage. Please review.

Project coverage is 69.53%. Comparing base (56c39b2) to head (71256ec).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/engine/plan_description.go	91.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16802   +/-   ##
=======================================
  Coverage   69.53%   69.53%           
=======================================
  Files        1567     1567           
  Lines      202388   202401   +13     
=======================================
+ Hits       140723   140745   +22     
+ Misses      61665    61656    -9     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.