metrics: adjust the parse & compile duration metrics (pingcap#9925)

From current observation, parse cost more than 160us in general,
and compile may take long if it involves subquery or allocate table ID.
Adjust related metrics to be more accurate.

metrics/session.go

@@ -23,15 +23,16 @@ var (
 			Subsystem: "session",
 			Name:      "parse_duration_seconds",
 			Help:      "Bucketed histogram of processing time (s) in parse SQL.",
-			Buckets:   prometheus.LinearBuckets(0.00004, 0.00001, 13),
+			Buckets:   prometheus.ExponentialBuckets(0.00004, 1.6, 13), // 40us ~ 18ms
 		}, []string{LblSQLType})
 	SessionExecuteCompileDuration = prometheus.NewHistogramVec(
 		prometheus.HistogramOpts{
 			Namespace: "tidb",
 			Subsystem: "session",
 			Name:      "compile_duration_seconds",
 			Help:      "Bucketed histogram of processing time (s) in query optimize.",
-			Buckets:   prometheus.LinearBuckets(0.00004, 0.00001, 13),
+			// Build plan may execute the statement, or allocate table ID, so it might take a long time.
+			Buckets: prometheus.ExponentialBuckets(0.00004, 2, 13), // 40us ~ 327ms
 		}, []string{LblSQLType})
 	SessionExecuteRunDuration = prometheus.NewHistogramVec(
 		prometheus.HistogramOpts{