What is the Metadata GC Threshold and how do I tune it?

While there is already an accepted answer, wanted to mention that there is also :

-XX:MaxMetaspaceSize=<NNN> where <NNN> is the maximum amount of space to be allocated for class metadata (in bytes).

Also from here,

Garbage collection of the dead classes and classloaders is triggered once the class metadata usage reaches the “MaxMetaspaceSize”.

There is a list of available options in this post.

The log message tells that GC was caused by Metaspace allocation failure. Metaspaces hold class metadata. They have appeared in Java 8 to replace PermGen.

Here are some options to tune Metaspaces.
You may want to set one or several of the following options:

-XX:MetaspaceSize=100M Sets the size of the allocated class metadata space that will trigger a garbage collection the first time it is exceeded;

-XX:InitialBootClassLoaderMetaspaceSize=32M to increase the boot class loader Metaspace;

-XX:MinMetaspaceFreeRatio=50 to make Metaspaces grow more agressively;

-XX:MaxMetaspaceFreeRatio=80 to reduce the chance of Metaspaces shrinking;

-XX:MinMetaspaceExpansion=4M the minumum size by which a Metaspace is exanded;

-XX:MaxMetaspaceExpansion=16M the maximum size to expand a Metaspace by without Full GC.