@Nullable
    public static String headerValueCacheSpec() {
        return HEADER_VALUE_CACHE_SPEC;
    }
Returns the {@linkplain CaffeineSpec Caffeine specification string} of the cache that stores the recent
results for converting a raw HTTP ASCII header value into a {@link String}. Only the header values
whose corresponding header name is listed in {@link #cachedHeaders()} will be cached.

<p>The default value of this flag is {@value DefaultFlagsProvider#HEADER_VALUE_CACHE_SPEC}. Specify the
{@code -Dcom.linecorp.armeria.headerValueCache=<spec>} JVM option to override the default value.
For example, {@code -Dcom.linecorp.armeria.headerValueCache=maximumSize=4096,expireAfterAccess=600s}.
Also, specify {@code -Dcom.linecorp.armeria.headerValueCache=off} JVM option to disable it.
core/src/main/java/com/linecorp/armeria/common/Flags.java