finagle-stats: convert illegal Prometheus characters to underscores

==== Problem
Finagle's Prometheus exporter will happily export hyphens and forward slashes which are illegal Prometheus characters.
==== Solution
Replace all illegal characters with underscores

Differential Revision: https://phabricator.twitter.biz/D1218924

Author: keremnc

finagle-stats-core/src/main/scala/com/twitter/finagle/stats/PrometheusExporter.scala

@@ -17,6 +17,7 @@ private final class PrometheusExporter(
   exportMetadata: Boolean,
   exportEmptyQuantiles: Boolean,
   verbosityPattern: Option[String => Boolean]) {
+  private[this] final val NamePattern = """[^a-zA-Z0-9_]""".r
 
   /**
    * Write the value of the metric as a `Double`.
@@ -162,7 +163,7 @@ private final class PrometheusExporter(
           } else {
             writer.append(',')
           }
-          writer.append(name)
+          writer.append(NamePattern.replaceAllIn(name, MetricBuilder.DimensionalNameScopeSeparator))
           writer.append('=')
           writer.append('"')
           writer.append(value)
@@ -249,7 +250,9 @@ private final class PrometheusExporter(
   }
 
   private[this] def writeName(writer: StringBuilder, name: Seq[String]): Unit =
-    name.addString(writer, MetricBuilder.DimensionalNameScopeSeparator)
+    name.view
+      .map { part => NamePattern.replaceAllIn(part, MetricBuilder.DimensionalNameScopeSeparator) }
+      .addString(writer, MetricBuilder.DimensionalNameScopeSeparator)
 
   /**
    * Write user-defined labels and reserved labels for Prometheus summary and histogram.

finagle-stats-core/src/test/scala/com/twitter/finagle/stats/PrometheusExporterTest.scala

@@ -261,4 +261,24 @@ class PrometheusExporterTest extends AnyFunSuite {
           |foo_ping_ms_sum{biz="bar"} 6
           |""".stripMargin)
   }
+
+  test("translate illegal Prometheus characters to underscores") {
+    val counter = CounterSnapshot(
+      hierarchicalName = "metric",
+      builder = MetricBuilder(
+        name = Seq("invalid-metric.seconds"),
+        metricType = CounterType,
+        units = Seconds
+      ).withLabels(Map("invalid.label" -> "foo")).withDimensionalSupport,
+      value = 500L
+    )
+    val result = writeMetrics(Seq(counter), Seq.empty, Seq.empty, exportMetadata = true)
+
+    assert(
+      result ==
+        """# TYPE invalid_metric_seconds counter
+          |# UNIT invalid_metric_seconds Seconds
+          |invalid_metric_seconds{invalid_label="foo"} 500
+          |""".stripMargin)
+  }
 }