Extend/refine information included in healthcheck payload

[jimmie]

💡 Description

Currently, the healthcheck payload includes: # of documents (CCS included), # of shards (including successful, skipped and error), raw JVM memory numbers (as returned by the Runtime class) and any encountered error messages (presence of which transitions return code from 200 to 418 (I_AM_A_TEAPOT).

Improvements to the information returned (for the sake of monitoring and operations since ECS itself ignores the payload):

• Compute more useful/meaningful JVM memory numbers - as described in the first answer here
• Opensearch used and available space via GET/_cat/indices, GET /_cat/shards and GET /_cat/allocations