Inefficient getDuration() parsing - Re-creation of Pattern

[baltiyskiy]

We've noticed when we are calling to Config.getDuration() often enough, it generates a lot of garbage. Allocation profiling shows the following picture:

void java.util.regex.Pattern$BitClass.<init>()	24139	97.3 %
Pattern$CharPredicate java.util.regex.Pattern.clazz(boolean)	24139	97.3 %
Pattern$Node java.util.regex.Pattern.sequence(Pattern$Node)	24139	97.3 %
Pattern$Node java.util.regex.Pattern.expr(Pattern$Node)	24139	97.3 %
void java.util.regex.Pattern.compile()	24139	97.3 %
void java.util.regex.Pattern.<init>(String, int)	24139	97.3 %
Pattern java.util.regex.Pattern.compile(String)	24139	97.3 %
boolean java.util.regex.Pattern.matches(String, CharSequence)	24139	97.3 %
boolean java.lang.String.matches(String)	24139	97.3 %
long com.typesafe.config.impl.SimpleConfig.parseDuration(String, ConfigOrigin, String)	24139	97.3 %
Duration com.typesafe.config.impl.SimpleConfig.getDuration(String)	24139	97.3 %

Also

void java.util.ArrayList.<init>(int)	2770	3.97 %
void java.util.regex.Pattern.compile()	2084	2.99 %
void java.util.regex.Pattern.<init>(String, int)	2084	2.99 %
Pattern java.util.regex.Pattern.compile(String)	2084	2.99 %
boolean java.util.regex.Pattern.matches(String, CharSequence)	2084	2.99 %
boolean java.lang.String.matches(String)	2084	2.99 %
long com.typesafe.config.impl.SimpleConfig.parseDuration(String, ConfigOrigin, String)	2084	2.99 %
Duration com.typesafe.config.impl.SimpleConfig.getDuration(String)	2084	2.99 %

Basically, SimpleConfig.getDuration() uses String.matches() API (https://github.com/lightbend/config/blob/main/config/src/main/java/com/typesafe/config/impl/SimpleConfig.java#L781), which, internally, creates a new Pattern object just to throw it away.
A simple fix is to hoist Pattern object as a static field and use it to perform the matching.