[lint] finagle: avoid implicit widening from integer division to floating point

In scala 2.12.20, it is a lint violation to perform an integer division that is widened to floating point like:
```
def compute(i: Int): Float = i / 2
```

Differential Revision: https://phabricator.twitter.biz/D1206654

Author: keremnc

finagle-exp/src/main/scala/com/twitter/finagle/exp/fiber_scheduler/util/Optimizer.scala

@@ -91,7 +91,7 @@ private[fiber_scheduler] final case class Optimizer(
         log.error(ex, "optimizer failure")
         stats.failure.incr()
     } finally {
-      stats.latency.add((System.nanoTime() - start) / 1000000)
+      stats.latency.add((System.nanoTime() - start) / 1000000f)
     }
   }
 

finagle-mux/src/main/scala/com/twitter/finagle/mux/lease/exp/ByteCounter.scala

@@ -74,8 +74,8 @@ private[lease] class WindowedByteCounter private[lease] (val info: JvmInfo, ctx:
   @volatile private[lease] var passCount: Int = 0
 
   /** @return allocation rate in bytes per millisecond. */
-  def rate(): Double = sum().inBytes / W.inMilliseconds
-  private[this] def lastRate(): Double = allocs(idx).inBytes / P.inMilliseconds
+  def rate(): Double = sum().inBytes / W.inMilliseconds.toDouble
+  private[this] def lastRate(): Double = allocs(idx).inBytes / P.inMilliseconds.toDouble
 
   override def toString =
     "WindowedByteCounter(windowed=" +

finagle-mux/src/test/scala/com/twitter/finagle/mux/lease/exp/WindowedByteCounterTest.scala

@@ -3,12 +3,16 @@ package com.twitter.finagle.mux.lease.exp
 import com.twitter.conversions.DurationOps._
 import com.twitter.util._
 import com.twitter.conversions.StorageUnitOps._
-import org.scalatest.concurrent.{Eventually, IntegrationPatience}
+import org.scalatest.concurrent.Eventually
+import org.scalatest.concurrent.IntegrationPatience
 import scala.util.Random
+import org.scalactic.Tolerance._
 import org.scalatest.funsuite.AnyFunSuite
 
 class WindowedByteCounterTest extends AnyFunSuite with Eventually with IntegrationPatience {
 
+  private[this] val epsilon = 1e-3
+
   trait ByteCounterHelper {
     val fakePool = new FakeMemoryPool(new FakeMemoryUsage(StorageUnit.zero, StorageUnit.zero))
     val fakeBean = new FakeGarbageCollectorMXBean(0, 0)
@@ -93,9 +97,9 @@ class WindowedByteCounterTest extends AnyFunSuite with Eventually with Integrati
           nextPeriod()
         }
 
-        assert(
-          counter.rate() == (WindowedByteCounter.N.kilobytes).inBytes / WindowedByteCounter.W.inMilliseconds
-        )
+        val expectedRate =
+          WindowedByteCounter.N.kilobytes.inBytes / WindowedByteCounter.W.inMilliseconds.toDouble
+        assert(counter.rate() === expectedRate +- epsilon)
     }
   }
 
@@ -111,9 +115,9 @@ class WindowedByteCounterTest extends AnyFunSuite with Eventually with Integrati
           nextPeriod()
         }
 
-        assert(
-          counter.rate() == (WindowedByteCounter.N.kilobytes).inBytes / WindowedByteCounter.W.inMilliseconds
-        )
+        val expectedRate1 =
+          WindowedByteCounter.N.kilobytes.inBytes / WindowedByteCounter.W.inMilliseconds.toDouble
+        assert(counter.rate() === expectedRate1 +- epsilon)
 
         for (i <- 1 to WindowedByteCounter.N) {
           fakePool.setSnapshot(
@@ -122,10 +126,9 @@ class WindowedByteCounterTest extends AnyFunSuite with Eventually with Integrati
           nextPeriod()
         }
 
-        assert(
-          counter
-            .rate() == (2 * (WindowedByteCounter.N.kilobytes).inBytes / WindowedByteCounter.W.inMilliseconds)
-        )
+        val expectedRate2 =
+          2 * WindowedByteCounter.N.kilobytes.inBytes / WindowedByteCounter.W.inMilliseconds.toDouble
+        assert(counter.rate() === expectedRate2 +- epsilon)
     }
   }
 
@@ -145,7 +148,8 @@ class WindowedByteCounterTest extends AnyFunSuite with Eventually with Integrati
           nextPeriod()
         }
 
-        assert(counter.rate() == x.inBytes / WindowedByteCounter.W.inMilliseconds)
+        val expectedRate = x.inBytes / WindowedByteCounter.W.inMilliseconds.toDouble
+        assert(counter.rate() === expectedRate +- epsilon)
     }
   }
 
@@ -176,9 +180,9 @@ class WindowedByteCounterTest extends AnyFunSuite with Eventually with Integrati
           nextPeriod()
         }
 
-        assert(
-          counter.rate() == WindowedByteCounter.N.kilobytes.inBytes / WindowedByteCounter.W.inMilliseconds
-        )
+        val expectedRate =
+          WindowedByteCounter.N.kilobytes.inBytes / WindowedByteCounter.W.inMilliseconds.toDouble
+        assert(counter.rate() === expectedRate +- epsilon)
     }
   }
 

finagle-netty4/src/main/scala/com/twitter/finagle/netty4/threading/EventLoopGroupTrackingRunnable.scala

@@ -124,7 +124,7 @@ private[threading] class EventLoopGroupTrackingRunnable(
     cpuTimeCounter.incr(TimeUnit.NANOSECONDS.toMillis(cpuTime))
     if (prevWallTimeNs != 0 && wallTimeNs != 0) {
       cpuUtilStat.add(
-        10000 * cpuTime / wallTimeNs
+        10000f * cpuTime / wallTimeNs
       )
     }
     prevCPUTimeNs = currCPUTimeNs