Docs: Point links in metrics-reporting.md to GitHub Java source

[manuzhang]

It looks from the doc that RESTMetricsReporter is available for users. However, it's not a public class. This PR makes it public and add some javadocs. It also fixes its javadoc missing issue.

[manuzhang]

cc @nastra @jbonofre @Fokko

[Fokko]

@manuzhang How I read the doc is:

• You either enable the RESTMetricsReporter through the property, no need to override the class.
• You implement a custom reporter by overriding the MetricsReporter, which is public.

[Fokko]

@manuzhang I see your comment in #10378. I don't think that the class being private is the underlying issue of the missing javadoc. For example, the RestCatalog doesn't have docs either: https://iceberg.apache.org/docs/javadoc/1.5.2/org/apache/iceberg/rest/RESTCatalog.html

[Fokko]

Hmm, the file is missing here: https://github.com/apache/iceberg/tree/javadoc/1.5.2/org/apache/iceberg/rest

[manuzhang]

RESTCatalog has javadoc at https://github.com/apache/iceberg/blob/javadoc/1.5.2/org/apache/iceberg/rest/RESTCatalog.html. It looks another issue when deploying to website.

[manuzhang]

Default level of generating javadoc is public and protected. RESTMetricsReporter is package private so its javadoc is not generated.

[manuzhang]

If we don't intend to make RESTMetricsReporter public, then we can just remove the javadoc link on "Metrics Reporting" page.

[Fokko]

@manuzhang I think that would make more sense, but I would love to get @nastra input here (he's currently on holiday).

[Fokko]

The RESTCatalog.html is there on the asf-site branch: https://github.com/apache/iceberg/blob/asf-site/javadoc/1.5.2/org/apache/iceberg/rest/RESTCatalog.html

[Fokko]

The correct url is: https://iceberg.apache.org/javadoc/1.5.2/org/apache/iceberg/rest/RESTCatalog.html. Looks like the paths are not correct (/docs/ is in there).

[Fokko]

The URL in the metrics-reporting.md does look similar to the URLs that are working, so probably it is failing because the class is not in the Javadoc. Thanks again for reporting this @manuzhang 👍

[nastra]

LGTM once CI passes