[receiver/postgresql] add basic query sample collection for postgresqlreceiver

[cuichenli]

Description

we introduced the top query collection to postgresql receiver. this will collect the queries from pg_stat_statements and report the related metrics. this will help the end user to identify which queries are most executed, and resources most used.

Example output:

resourceLogs:
  - resource:
      attributes: []
    scopeLogs:
      - logRecords:
          - attributes:
              - key: db.system.name
                value:
                  stringValue: postgresql
              - key: db.namespace
                value:
                  stringValue: postgres
              - key: db.query.text
                value:
                  stringValue: "select * from pg_stat_activity where id = ?"
              - key: postgresql.calls 
                value: 
                  intValue: 3
              - key: postgresql.rows 
                value:
                  intValue: 10
              - key: postgresql.shared_blks_dirtied
                value:
                  intValue: 1 
              - key: postgresql.shared_blks_hit
                value:
                  intValue: 2
              - key: postgresql.shared_blks_read
                value:
                  intValue: 3
              - key: postgresql.shared_blks_written
                value:
                  intValue: 4
              - key: postgresql.temp_blks_read
                value:
                  intValue: 5
              - key: postgresql.temp_blks_written
                value:
                  intValue: 6
              - key: postgresql.queryid 
                value: 
                  stringValue: "114514"
              - key: postgresql.rolname 
                value: 
                  stringValue: master
              - key: postgresql.total_exec_time 
                value: 
                  doubleValue: 1
              - key: postgresql.total_plan_time 
                value: 
                  doubleValue: 1
            body:
              stringValue: top query
        scope:
          name: github.com/open-telemetry/opentelemetry-collector-contrib/receiver/postgresqlreceiver
          version: 0.0.1

[atoulme]

is that version correct?

[atoulme]

do they need to be exported?

[atoulme]

should the defer call be before we collect?

[atoulme]

shouldn't this be a resource attribute?

[atoulme]

LGTM. Some small things to address.