Fix #91, fine tune decimal number formatting.

The expected string for `123_456_789.012_345 ` is `123456789.012345` and is NOT `1.23456789012345E8` as we generate today.
See https://github.com/influxdata/influxdb-java/blob/influxdb-java-2.5/src/main/java/org/influxdb/dto/Point.java#L321

src/main/java/org/jmxtrans/agent/influxdb/InfluxMetric.java

@@ -23,18 +23,35 @@
  */
 package org.jmxtrans.agent.influxdb;
 
+import java.math.BigDecimal;
+import java.text.DecimalFormat;
+import java.text.NumberFormat;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 import java.util.Objects;
 
 import org.jmxtrans.agent.util.StringUtils2;
 
+import javax.annotation.concurrent.NotThreadSafe;
+
 /**
  * @author Kristoffer Erlandsson
  */
 public class InfluxMetric {
 
     private static final String FIELD_NAME = "value";
+
+    /*
+     * See https://github.com/influxdata/influxdb-java/blob/influxdb-java-2.5/src/main/java/org/influxdb/dto/Point.java#L321
+     */
+    protected final static NumberFormat NUMBER_FORMAT;
+    static {
+        NUMBER_FORMAT = NumberFormat.getInstance(Locale.ENGLISH);
+        NUMBER_FORMAT.setMaximumFractionDigits(340);
+        NUMBER_FORMAT.setGroupingUsed(false);
+        NUMBER_FORMAT.setMinimumFractionDigits(1);
+    }
     private final long timestampMillis;
     private final List<InfluxTag> tags;
     private final String measurement;
@@ -83,6 +100,11 @@ private String valueAsStr() {
         if (value instanceof Integer || value instanceof Long) {
             return value.toString() + "i";
         }
+        if (value instanceof Float || value instanceof Double || value instanceof BigDecimal) {
+            synchronized (NUMBER_FORMAT) {
+                return NUMBER_FORMAT.format(value);
+            }
+        }
         return value.toString();
     }
 

src/test/java/org/jmxtrans/agent/influxdb/InfluxMetricConverterTest.java

@@ -90,7 +90,7 @@ public void toInfluxFormatInteger() throws Exception {
     @Test
     public void toInfluxFormatBigFloat() throws Exception {
         InfluxMetric metric = new InfluxMetric("foo", EMPTY_TAG_LIST, 123_456_789.012_345, 2123l);
-        assertThat(metric.toInfluxFormat(), equalTo("foo value=1.23456789012345E8 2123"));
+        assertThat(metric.toInfluxFormat(), equalTo("foo value=123456789.012345 2123"));
     }
 
     @Test