HDDS-14807. [Docs] Erasure Coding related metrics.

[jojochuang]

What changes were proposed in this pull request?

HDDS-14807. [Docs] Erasure Coding related metrics.

Please describe your PR in detail:

• Add the metrics related to Erasure Coding. to Core Concepts -> Replication -> Erasure Coding
• Generated by the following three prompts using Gemini Cli + Gemini 2.5 Flash at Ozone's main repository:

> Find any metrics that is related to erasure coding. All metrics are defined in files that end with Metrics.java, for example,
   @hadoop-hdds/container-service/src/main/java/org/apache/hadoop/ozone/container/ec/reconstruction/ECReconstructionMetrics.java

   To determine if a metric is related to erasure coding, find the variable name, the description that has "ec" or "erasurecoding"

> Add to the bottom of @hadoop-hdds/docs/content/feature/ErasureCoding.md

   These metrics span across Datanode, SCM and client.  Useful for troubleshooting EC problems. For each metric, create a table and include metric name
   in JMX, in Prometheus, and a description of the metric. Categorize them based on the service roles: Datanode, SCM or client.

> @hadoop-hdds/docs/content/feature/ErasureCoding.md the Prometheus metric names need correction.

   For example:

   scm_replication_manager_metrics_inflight_ec_replication --> replication_manager_metrics_inflight_ec_replication

   That is, remove the "scm_" prefix.

   Similarly, datanode_ec_reconstruction_metrics_block_group_reconstruction_total --> ec_reconstruction_metrics_block_group_reconstruction_total

   Remove the "datanode_" prefix.

   Finally, client_xceiver_client_metrics_ec_reconstruction_total --> xceiver_client_metrics_ec_reconstruction_total. Remove the "client_" prefix.

• Copy and paste to ozone-site repo. Manually verified the content correctness.

What is the link to the Apache Jira?

https://issues.apache.org/jira/browse/HDDS-14807

How was this patch tested?

Please explain how this patch was tested. In most cases this will just be checking the local preview of the website, but existing CI checks should also pass.

[jojochuang]

cc @ptlrs