Use StringBuilder.append(char) where possible

implementations/micrometer-registry-appoptics/src/main/java/io/micrometer/appoptics/AppOpticsMeterRegistry.java

@@ -267,7 +267,7 @@ private String write(Meter.Id id, @Nullable String type, String... statistics) {
             // appoptics requires at least one tag for every metric, so we hang something here that may be useful.
             sb.append("\"_type\":\"").append(type).append('"');
             if (!tags.isEmpty())
-                sb.append(",");
+                sb.append(',');
         }
 
         if (!tags.isEmpty()) {

implementations/micrometer-registry-elastic/src/main/java/io/micrometer/elastic/ElasticMeterRegistry.java

@@ -430,7 +430,7 @@ String writeDocument(Meter meter, Consumer<StringBuilder> consumer) {
         }
 
         consumer.accept(sb);
-        sb.append("}");
+        sb.append('}');
 
         return sb.toString();
     }

implementations/micrometer-registry-graphite/src/main/java/io/micrometer/graphite/GraphiteHierarchicalNameMapper.java

@@ -43,7 +43,7 @@ public String toHierarchicalName(Meter.Id id, NamingConvention convention) {
         for (String tagKey : tagsAsPrefix) {
             String tagValue = id.getTag(tagKey);
             if (tagValue != null) {
-                hierarchicalName.append(convention.tagValue(tagValue)).append(".");
+                hierarchicalName.append(convention.tagValue(tagValue)).append('.');
             }
         }
         hierarchicalName.append(id.getConventionName(convention));

implementations/micrometer-registry-stackdriver/src/main/java/io/micrometer/stackdriver/StackdriverMeterRegistry.java

@@ -414,7 +414,7 @@ private void prePopulateVerifiedDescriptors() {
         private String metricType(Meter.Id id, @Nullable String statistic) {
             StringBuilder metricType = new StringBuilder("custom.googleapis.com/").append(getConventionName(id));
             if (statistic != null) {
-                metricType.append("/").append(statistic);
+                metricType.append('/').append(statistic);
             }
             return metricType.toString();
         }

micrometer-core/src/main/java/io/micrometer/core/ipc/http/HttpSender.java

@@ -107,7 +107,7 @@ public static Builder build(String uri, HttpSender sender) {
 
         @Override
         public String toString() {
-            StringBuilder printed = new StringBuilder(method.toString()).append(" ")
+            StringBuilder printed = new StringBuilder(method.toString()).append(' ')
                     .append(url.toString()).append("\n");
             if (entity.length == 0) {
                 printed.append("<no request body>");