[LIVY-1070] Add LivySessionMetrics Codahale gauges for session monitoring

## What changes were proposed in this pull request?

This PR adds server-side session monitoring gauges for LIVY-1070.

Problem: Livy exposes session state via the REST API (/sessions, /batches) but does not publish session counts as Codahale metrics on the existing /metrics endpoint. External monitoring systems must poll the REST API to observe session distribution.

Solution: Introduce LivySessionMetrics, which registers session count gauges into Livy's MetricRegistry at server startup. Gauges are derived from InteractiveSessionManager and BatchSessionManager and exposed alongside existing Livy metrics via the AdminServlet at /metrics.

Metrics registered (18 gauges)

Overall (3)

    livy.sessions.total
    livy.sessions.active.total
    livy.sessions.terminal.total

Interactive (8)

    livy.sessions.interactive.total
    livy.sessions.interactive.{idle,busy,starting,shutting_down,dead,error,killed}

Batch (7)

    livy.sessions.batch.total
    livy.sessions.batch.{starting,running,success,dead,error,killed}

Design notes

    Additive only — no REST API or session lifecycle behavior changes
    Idempotent registration — skips gauge names already present in the registry
    Error-safe callbacks — gauge getValue returns 0 on exception
    State normalization — handles case and hyphen/underscore variants (e.g. shuttingdown, succeeded)
    HA note — gauge values reflect the local Livy server instance; in HA deployments only the leader holds active sessions

Compatibility

    No new endpoints; uses existing /metrics AdminServlet
    Backward compatible — new gauges appear alongside existing metrics

## How was this patch tested?

Build
mvn package -Pspark3 -Pscala-2.12 -pl server -am \
-s /tmp/livy-mvn-central-settings.xml -DskipTests

Result: BUILD SUCCESS
Unit tests
mvn test -Pspark3 -Pscala-2.12 -pl server \
-s /tmp/livy-mvn-central-settings.xml \
-Dsuites=org.apache.livy.server.LivySessionMetricsSpec

Result: 8/8 tests passed

Test | Coverage -- | -- All 18 gauge registrations | Registration Duplicate registration guard | Idempotency Zero sessions | Empty state Interactive sessions by state | State counting Batch sessions by state (incl. succeeded alias) | State counting Batch succeeded alias | Edge case Overall totals (total, active, terminal) | Aggregation Exception fallback returns 0 | Error handling
Code coverage

JaCoCo agent enabled during test run (server/target/jacoco/main.exec generated).

No UI changes in this PR.

## Was this patch authored or co-authored using generative AI tooling?

Yes, this was co-authored using Cursor to help generate the new test cases.