RFR: 8307298: JFR: Ensure jdk.jfr.internal.TypeLibrary is initialized only once

[Robert Toyonaga]

Hello, may I have a review of a small change that enforces that `jdk.jfr.internal.TypeLibrary` only gets initialized once?
Previously, it was the case that `TypeLibrary` was only initialized once, but since [this change](https://github.com/openjdk/jdk/pull/12884/files) it can now be initialized multiple times. This can be a problem because re-initialization [results in attempting to add elements to `jdk.jfr.internal.Type#fields`](https://github.com/openjdk/jdk/blob/master/src/jdk.jfr/share/classes/jdk/jfr/internal/MetadataLoader.java#L264) which may have already been changed to an unmodifiableList after the initial initialization of `TypeLibrary`.

This is of concern for JFR support in GraalVM Native Image. Specifically, multiple initializations of TypeLibrary breaks the ability to use JFR at native image built time. This is because, if JFR is used at image build time, the data of a few of the JFR singletons such as MetadataRepository must be re-initialized for runtime recordings to start with a "clean-slate". However, re-initializing MetadataRepository, also has the effect of calling `TypeLibrary#initialize()` which now unconditionally attempts to initialize TypeRepository, resulting in errors.

Example of exception upon second initialization of `TypeRepository`: 

Caused by: java.lang.InternalError: java.lang.UnsupportedOperationException
	at jdk.jfr/jdk.jfr.internal.MetadataLoader.createTypes(MetadataLoader.java:195)
	at jdk.jfr/jdk.jfr.internal.TypeLibrary.initialize(TypeLibrary.java:103)
	at jdk.jfr/jdk.jfr.internal.MetadataRepository.initializeJVMEventTypes(MetadataRepository.java:72)
	at jdk.jfr/jdk.jfr.internal.MetadataRepository.<init>(MetadataRepository.java:68)
	at org.graalvm.nativeimage.builder/com.oracle.svm.core.jfr.Target_jdk_jfr_internal_MetadataRepository.<clinit>(Target_jdk_jfr_internal_MetadataRepository.java:42)
	at java.base/jdk.internal.misc.Unsafe.ensureClassInitialized0(Native Method)
	at java.base/jdk.internal.misc.Unsafe.ensureClassInitialized(Unsafe.java:1160)
	at org.graalvm.nativeimage.builder/com.oracle.svm.hosted.classinitialization.ClassInitializationSupport.ensureClassInitialized(ClassInitializationSupport.java:168)
	... 11 more
Caused by: java.lang.UnsupportedOperationException
	at java.base/java.util.Collections$UnmodifiableCollection.add(Collections.java:1085)
	at jdk.jfr/jdk.jfr.internal.Type.add(Type.java:227)
	at jdk.jfr/jdk.jfr.internal.MetadataLoader.addFields(MetadataLoader.java:264)
	at jdk.jfr/jdk.jfr.internal.MetadataLoader.buildTypes(MetadataLoader.java:202)
	at jdk.jfr/jdk.jfr.internal.MetadataLoader.createTypes(MetadataLoader.java:193)

-------------

Commit messages:
 - Only init TypeLibrary once

Changes: https://git.openjdk.org/jdk/pull/13760/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13760&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307298
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13760.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13760/head:pull/13760

PR: https://git.openjdk.org/jdk/pull/13760