Consider providing Micrometer integration

[vpavic]

We could consider exposing some session related metrics using Micrometer.

[anessi]

When enabling Spring Session (e.g. with Redis) on a Spring Boot app using embedded Tomcat one can notice that the tomcat.sessions.* metrics no longer get updated. These values are very useful for monitoring the application. These are the available metrics in tomcat:

tomcat.sessions.active.current
tomcat.sessions.active.max
tomcat.sessions.alive.max
tomcat.sessions.created
tomcat.sessions.expired
tomcat.sessions.rejected

It would be very nice to get similar metrics from Spring Session.

---

Below two example stack traces for how the tomcat.sessions.created metric is set.

Without Spring Sessions:

createSession:725, ManagerBase (org.apache.catalina.session)
doGetSession:3093, Request (org.apache.catalina.connector)
getSession:2493, Request (org.apache.catalina.connector)
getSession:908, RequestFacade (org.apache.catalina.connector)
getSession:920, RequestFacade (org.apache.catalina.connector)
getSession:249, HttpServletRequestWrapper (javax.servlet.http)
...
doRequest:193, LoginServlet (xxxxxx.servlet)
doGet:53, BaseServlet (xxxxxx.servlet)
service:645, HttpServlet (javax.servlet.http)
...

With Spring Session enabled (spring-session-data-redis:2.6.0):

createSession:509, RedisIndexedSessionRepository (org.springframework.session.data.redis)
createSession:251, RedisIndexedSessionRepository (org.springframework.session.data.redis)
getSession:323, SessionRepositoryFilter$SessionRepositoryRequestWrapper (org.springframework.session.web.http)
getSession:332, SessionRepositoryFilter$SessionRepositoryRequestWrapper (org.springframework.session.web.http)
getSession:193, SessionRepositoryFilter$SessionRepositoryRequestWrapper (org.springframework.session.web.http)
getSession:249, HttpServletRequestWrapper (javax.servlet.http)
...
doRequest:193, LoginServlet (xxxxxx.servlet)
doGet:53, BaseServlet (xxxxxx.servlet)
service:645, HttpServlet (javax.servlet.http)

The number of created sessions provided by the metric tomcat.sessions.created is stored on the ManagerBase class and retrieved using:

getSessionCounter:944, ManagerBase (org.apache.catalina.session)
applyAsDouble:-1, 1596426482 (io.micrometer.core.instrument.binder.tomcat.TomcatMetrics$$Lambda$1376)
count:39, CumulativeFunctionCounter (io.micrometer.core.instrument.cumulative)
get:-1, 49082497 (io.micrometer.core.instrument.FunctionCounter$$Lambda$1546)
getValue:39, Measurement (io.micrometer.core.instrument)
lambda$mergeMeasurements$5:131, MetricsEndpoint (org.springframework.boot.actuate.metrics)
...
accept:-1, 1871920265 (org.springframework.boot.actuate.metrics.MetricsEndpoint$$Lambda$1545)
...