add escaping for backslashes, fixes influxdata#718

Author: oliverrahner

client/src/main/java/com/influxdb/client/write/Point.java

@@ -442,6 +442,9 @@ private void escapeKey(@Nonnull final StringBuilder sb, @Nonnull final String ke
     private void escapeKey(@Nonnull final StringBuilder sb, @Nonnull final String key, final boolean escapeEqual) {
         for (int i = 0; i < key.length(); i++) {
             switch (key.charAt(i)) {
+                case '\\':
+                    sb.append("\\\\");
+                    continue;
                 case '\n':
                     sb.append("\\n");
                     continue;

client/src/test/java/com/influxdb/client/write/PointTest.java

@@ -182,6 +182,16 @@ void fieldEscape() {
         Assertions.assertThat(point.toLineProtocol()).isEqualTo("h2o,location=europe level=\"string esc\\\"ape value\"");
     }
 
+    @Test
+    void tagEscape() {
+        Point point = Point.measurement("h2o")
+                .addTag("location", "\\")
+                .addTag("zone", "europe")
+                .addField("level", "dummy value");
+
+        Assertions.assertThat(point.toLineProtocol()).isEqualTo("h2o,location=\\\\,zone=europe level=\"dummy value\"");
+    }
+
     @Test
     void time() {