Simple bypass for servlet spec < 3.x

[SylvainJuge]

What does this PR do?

This pull-request modifies Servlet plugin instrumentation to make it gracefully ignore Servlet implementations before Servlet API 3.0.

Without this change, users get spammed in application server logs with similar exception (Here on Tomcat 6.x):

java.lang.NoSuchMethodError: javax.servlet.ServletRequest.getDispatcherType()Ljavax/servlet/DispatcherType; at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:181) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293) at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:859) at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:610) at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:503) at java.lang.Thread.run(Thread.java:748) 

While this change avoids spamming the server logs, there is no explicit warning message from this agent when an unsupported version is being used. Thus from user's perspective it will just make the agent not monitor any HTTP transaction (but JMX metrics will still be properly working as expected).

Checklist

• My code follows the style guidelines of this project
• I have rebased my changes on top of the latest master branch
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works (tested manually)
• New and existing unit tests pass locally with my changes
• I have updated CHANGELOG.asciidoc
• I have updated supported-technologies.asciidoc (no change required, previous versions were not supported).

Author's Checklist

• Manual testing with Servlet 2.5 container (Tomcat 6.x).
• Manual testing with Servlet 3.0 container (Tomcat 7.x or later).
• Decide if we want to have a proper warning when such an incompatible Servlet version is used (currently we silently ignore all transactions).

Related issues

• Closes Don't instrument Servlet API < 3.0 #894

[felixbarny]

Decide if we want to have a proper warning when such an incompatible Servlet version is used (currently we silently ignore all transactions).

Maybe we can couple this with logging javax.servlet.ServletContext#getServerInfo but that can be done in a separate advice.

[felixbarny]

Seems like tests fail on Payara

[SylvainJuge]

I've added a quick check on Servlet.init that will output a warning similar to this one

WARN co.elastic.apm.agent.servlet.ServletTransactionHelper - Unsupported servlet version detected: 2.5, no Servlet transaction will be created 

[felixbarny]

A few comments on the ServletInitInstrumentation but we can also merge the PR without this instrumentation.

[felixbarny]

To avoid a CAS on every invocation

Suggested change

	if (!doLogAndWarn.getAndSet(false)) {
	if (!doLogAndWarn.get() || !doLogAndWarn.getAndSet(false)) {

maybe rename doLogAndWarn to alreadyLogged to get rid of the negations which are a bit harder to parse?

[SylvainJuge]

definitely agree on having less negations, however doing a double check here seems a bit overkill. I think it's fine to replace this AtomicBoolean with a simple volatile boolean as AtomicBoolean.get does makes a volatile read, thus it won't be more efficient.