Add perf_schema quantile columns to collector

[matthewnolf]

This change appends the 3 quantile columns exposed in the events_statements_summary_by_digest table to the
perf_schema_events_statements collector so that they are available as metrics.

We believe this to be a useful addition so that the various quantiles (p95, p99, p99.5) are available to query and spot changes in performance for a given query. Our use case is to visualise these quantiles for a specific digest to provide more insight than just average query times.

There is mention of adding quantiles in a previous change from a colleague here which wasn't further investigated.

Sample /metrics output that includes the new metrics:

# HELP mysql_perf_schema_events_statements_quantile_95 The 95th percentile of the statement latency, in picoseconds.
# TYPE mysql_perf_schema_events_statements_quantile_95 counter
mysql_perf_schema_events_statements_quantile_95{digest="12fbbd91beb73676bfc7c0cf3cc73d8c29ccc0349a54e367556b6fb7d244a4f8",digest_text="SELECT DISTINCTROW `stack_id` FROM `jobs`",schema="hosted_exporters"} 2.08929613e+08

# HELP mysql_perf_schema_events_statements_quantile_99 The 99th percentile of the statement latency, in picoseconds.
# TYPE mysql_perf_schema_events_statements_quantile_99 counter
mysql_perf_schema_events_statements_quantile_99{digest="12fbbd91beb73676bfc7c0cf3cc73d8c29ccc0349a54e367556b6fb7d244a4f8",digest_text="SELECT DISTINCTROW `stack_id` FROM `jobs`",schema="hosted_exporters"} 2.39883291e+08

# HELP mysql_perf_schema_events_statements_quantile_999 The 99.9th percentile of the statement latency, in picoseconds.
# TYPE mysql_perf_schema_events_statements_quantile_999 counter
mysql_perf_schema_events_statements_quantile_999{digest="12fbbd91beb73676bfc7c0cf3cc73d8c29ccc0349a54e367556b6fb7d244a4f8",digest_text="SELECT DISTINCTROW `stack_id` FROM `jobs`",schema="hosted_exporters"} 1.819700858e+09

[cristiangreco]

@SuperQ could use your thoughts here :)