Fix Index size metric with correct values

[iserko]

At my company we just used this to check a specific table in our database and noticed that the index_size was wrong.
Turns out the query used to pull that information is slightly off.

According to PostgreSQL docs https://www.postgresql.org/docs/current/catalog-pg-index.html
indrelid is The OID of the pg_class entry for the table this index is for
since the indexoid is used to calculate size by giving it as the parameter to pg_relation_size, that returns the size of the whole table and not just the index.

The fix is to use indexrelid which is The OID of the pg_class entry for this index.

[CLAassistant]

All committers have signed the CLA.

[carlossscastro]

This has been tested on both versions to validate index size values

Output of both integration versions:
currentversion_output.txt
fixedversion_output.txt

Output from Postgres DB:
postgresdb_output.txt

Compare example:

Validation Version:

      "metrics": [
        {
          "database": "demo",
          "displayName": "pg_attribute_relid_attnam_index",
          "entityName": "index:pg_attribute_relid_attnam_index",
          "event_type": "PostgresqlIndexSample",
          "index.sizeInBytes": 442368,
          "schema": "pg_catalog",
          "table": "pg_attribute"
        }
      ],

Fixed Version:

      "metrics": [
        {
          "database": "demo",
          "displayName": "pg_attribute_relid_attnam_index",
          "entityName": "index:pg_attribute_relid_attnam_index",
          "event_type": "PostgresqlIndexSample",
          "index.sizeInBytes": 131072,
          "schema": "pg_catalog",
          "table": "pg_attribute"
        }
      ],

Output From Postgres:

|                Name                 | Type  |  Owner   |      Table      | Persistence |    Size    |
| pg_attribute_relid_attnam_index     | index | postgres | pg_attribute    | permanent   | 128 kB     |

[carlossscastro]

@iserko
Thanks a lot for spotting this and for your contribution
We will be releasing this in the next few days.