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 vitess-tester#21

[vitess-bot]

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[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     

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