fix(iota-indexer): add fallback query to get total packages metric

[kodemartin]

Description of change

This patch adds a fallback query to get the total packages metric.

The primary query relies on the pg_class catalog and uses the reltuples field to get a fast estimate of the row count. However reading the documentation

reltuples float4

Number of live rows in the table. This is only an estimate used by the planner. It is updated by VACUUM, ANALYZE, and a few DDL commands such as CREATE INDEX. If the table has never yet been vacuumed or analyzed, reltuples contains -1 indicating that the row count is unknown.

It has been observed that for a vanilla network that starts with only the system packages, the query for the estimate returns -1, likely because the insert threshold for triggering vacuuming was not supassed (see autovacuum).

Credits to @begonaalvarezd for observing the issue.

Type of change

• Bug fix (a non-breaking change which fixes an issue)

How the change has been tested

Run a local network with vanilla genesis and the indexer rpc and run the following query:

curl 'http://0.0.0.0:9005/'   -H 'content-type: application/json' --data-raw '{"jsonrpc":"2.0","id":3,"method":"iotax_getNetworkMetrics","params":[]}' 

Change checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes

[samuel-rufi]

☀️