trace_methods needs some definition at startup or redefining later fails

## Describe the bug
[trace_methods](https://www.elastic.co/guide/en/apm/agent/java/current/config-core.html#config-trace-methods) is dynamic, but it looks like it needs to be defined with SOMETHING at startup, then it can successfully be redefined. But if not defined at all at startup, the redefinition fails with mapping errors
```
java.lang.IllegalStateException: There's no mapping for key co.elastic.apm.agent.tracemethods.TraceMethodInstrumentation$TraceMethodAdvice
```

## Steps to reproduce
Do NOT define trace_methods at startup (ie it should be absent from properties and config files), then sometime after startup define it, eg
```
new Thread(() -> {
try {Thread.sleep(30_000L);} catch (InterruptedException e) {}
System.setProperty("elastic.apm.trace_methods", "somepackage.*");
}).start();
```

## Expected behavior
For that example, all methods in `somepackage` should produce no transactions until about 30 seconds after startup, when it should start producing transactions for any method executed in `somepackage`

## Debug logs
```
2023-08-21 14:48:06,614 [Thread-16] ERROR co.elastic.apm.agent.bci.IndyBootstrap - There's no mapping for key co.elastic.apm.agent.tracemethods.TraceMethodInstrumentation$TraceMethodAdvice
java.lang.IllegalStateException: There's no mapping for key co.elastic.apm.agent.tracemethods.TraceMethodInstrumentation$TraceMethodAdvice
at co.elastic.apm.agent.bci.ElasticApmAgent.getInstrumentationClassLoader(ElasticApmAgent.java:946) ~[elastic-apm-agent-1.38.1-SNAPSHOT.jar:1.38.1-SNAPSHOT]
at co.elastic.apm.agent.bci.IndyBootstrap.internalBootstrap(IndyBootstrap.java:416) ~[elastic-apm-agent-1.38.1-SNAPSHOT.jar:1.38.1-SNAPSHOT]
at co.elastic.apm.agent.bci.IndyBootstrap.bootstrap(IndyBootstrap.java:389) ~[elastic-apm-agent-1.38.1-SNAPSHOT.jar:1.38.1-SNAPSHOT]
at jdk.internal.reflect.GeneratedMethodAccessor16.invoke(Unknown Source) ~[?:?]
at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?]
at java.lang.reflect.Method.invoke(Method.java:568) ~[?:?]
at java.lang.IndyBootstrapDispatcher.bootstrap(IndyBootstrapDispatcher.java:61) ~[?:?]
at java.lang.invoke.BootstrapMethodInvoker.invoke(BootstrapMethodInvoker.java:147) ~[?:?]
at java.lang.invoke.CallSite.makeSite(CallSite.java:315) ~[?:?]
at java.lang.invoke.MethodHandleNatives.linkCallSiteImpl(MethodHandleNatives.java:281) ~[?:?]
at java.lang.invoke.MethodHandleNatives.linkCallSite(MethodHandleNatives.java:271) ~[?:?]
at wimtjgf.ScoreUI.getGcTime(ScoreUI.java:215) ~[bin/:?]
at wimtjgf.ScoreUI.lambda$2(Unknown Source) ~[bin/:?]
at java.lang.Thread.run(Thread.java:833) [?:?]
```

## Workaround

Add a dummy trace_methods definition at startup, eg `-Delastic.apm.trace_methods=nothing.at.all` then redefine dynamically as desired