add stmtType to ast.Prepared to record metric

[lysu]

What problem does this PR solve?

rel pingcap/tidb#11753

add stmtType in Prepared so we can calculate stmtType just in COM_PREPARE

What is changed and how it works?

RT

Check List

Tests

• Unit test
• Integration test

Code changes

• n/a

Side effects

• n/a

Related changes

• Need to cherry-pick to the release branch

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This change is 

[codecov]

Codecov Report

Merging #487 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #487      +/-   ##
==========================================
- Coverage   71.48%   71.44%   -0.04%     
==========================================
  Files          32       32              
  Lines        7672     7648      -24     
==========================================
- Hits         5484     5464      -20     
+ Misses       1667     1665       -2     
+ Partials      521      519       -2

Impacted Files	Coverage Δ
ast/misc.go	74.05% <ø> (ø)	⬆️
ast/ddl.go	79.85% <0%> (-0.06%)	⬇️
parser.go	70.58% <0%> (ø)	⬆️
misc.go	96.42% <0%> (ø)	⬆️

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[zz-jason]

LGTM

[kennytm]

LGTM