update based on comments

Author: yyanyy

api/src/main/java/org/apache/iceberg/FieldMetrics.java

@@ -47,27 +47,45 @@ public FieldMetrics(int id,
     this.upperBound = upperBound;
   }
 
-  public int getId() {
+  /**
+   * Returns the id of the field that the metrics within this class are associated with.
+   */
+  public int id() {
     return id;
   }
 
-  public long getValueCount() {
+  /**
+   * Returns the number of all values, including nulls, NaN and repeated, for the given field.
+   */
+  public long valueCount() {
     return valueCount;
   }
 
-  public long getNullValueCount() {
+  /**
+   * Returns the number of null values for this field.
+   */
+  public long nullValueCount() {
     return nullValueCount;
   }
 
-  public long getNanValueCount() {
+  /**
+   * Returns the number of NaN values for this field. Will only be non-0 if this field is a double or float field.
+   */
+  public long nanValueCount() {
     return nanValueCount;
   }
 
-  public ByteBuffer getLowerBound() {
+  /**
+   * Returns the lower bound value of this field.
+   */
+  public ByteBuffer lowerBound() {
     return lowerBound;
   }
 
-  public ByteBuffer getUpperBound() {
+  /**
+   * Returns the upper bound value of this field.
+   */
+  public ByteBuffer upperBound() {
     return upperBound;
   }
 }

api/src/main/java/org/apache/iceberg/Metrics.java

@@ -44,7 +44,10 @@ public class Metrics implements Serializable {
   public Metrics() {
   }
 
-  // for temporary backward compatibility, will be removed when all writers support nanValueCounts
+  /**
+   * @deprecated will be removed in 0.12.0; use {@link #Metrics(Long, Map, Map, Map, Map)} instead.
+   */
+  @Deprecated
   public Metrics(Long rowCount,
                  Map<Integer, Long> columnSizes,
                  Map<Integer, Long> valueCounts,
@@ -67,7 +70,10 @@ public Metrics(Long rowCount,
     this.nanValueCounts = nanValueCounts;
   }
 
-  // for temporary backward compatibility, will be removed when all writers support nanValueCounts
+  /**
+   * @deprecated will be removed in 0.12.0; use {@link #Metrics(Long, Map, Map, Map, Map, Map, Map)} instead.
+   */
+  @Deprecated
   public Metrics(Long rowCount,
                  Map<Integer, Long> columnSizes,
                  Map<Integer, Long> valueCounts,
@@ -135,7 +141,7 @@ public Map<Integer, Long> nullValueCounts() {
   }
 
   /**
-   * Get the number of NaN values for all fields in a file.
+   * Get the number of NaN values for all float and double fields in a file.
    *
    * @return a Map of fieldId to the number of NaN counts
    */