Add max, mean, and stddev timing metrics to stat_statements collector

[irfanhabib]

Motivation

The current pg_stat_statements collector only exports total execution time, which makes it difficult to understand query performance patterns. These new metrics allow users to:

• Identify queries with high execution time variance
• Detect performance regressions in individual query runs
• Better understand query performance distribution beyond just totals
• Set more informed alerting thresholds based on typical vs. maximum execution times

Changes

• Added three new Prometheus metric descriptors for max, mean, and standard deviation timing
• Updated all SQL query templates to include the appropriate columns for different PostgreSQL versions:
  • PostgreSQL < 13: max_time, mean_time, stddev_time
  • PostgreSQL 13+: max_exec_time, mean_exec_time, stddev_exec_time
• Modified the Update function to scan and emit the new metrics as gauge values
• Updated test cases to handle the additional columns

Compatibility

• Maintains backward compatibility with existing metrics
• Supports all PostgreSQL versions currently supported by the exporter
• Uses appropriate column names based on PostgreSQL version detection