Description of problem:
After installation of pcp-pmda-postgresql this PMDA does not collect statistics from a PostgreSQL instance due to a SELinux issue.

Version-Release number of selected component (if applicable):
pcp-5.0.2-5.el8
selinux-policy-3.14.3-41.el8

How reproducible:
Always

Steps to Reproduce:
1. Install RHEL-8.2 release
2. Install RPMs of pcp, pcp-pmda-postgresql, postgresql-server
3. Start the postgresql server and pmcd
4. Install PMDA
# cd /var/lib/pcp/pmdas/postgresql
# ./Install

Actual results:
* Installation throws errors:
[Mon Apr 20 10:21:08] pmdapostgresql(87409) Info: Connecting to host 'local', port '5432', db 'postgres' as user 'postgres'
[Mon Apr 20 10:21:08] pmdapostgresql(87409) Info: Error connecting to db postgres as user postgres: could not connect to server: No such file or directory
Is the server running locally and accepting
connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?

* Only one postgres metric is reported as active after the installation:
Check postgresql metrics have appeared ... 1 warnings, 1 metrics and 0 values

* /var/log/pcp/pmcd/postgresql.log contains error messages:
[Mon Apr 20 10:56:23] pmdapostgresql(64495) Info: Error connecting to db postgres as user postgres: could not connect to server: Permission denied
Is the server running locally and accepting
connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?

* "ausearch -m AVC" reports AVCs
type=AVC msg=audit(04/20/2020 10:15:43.559:1167) : avc:  denied  { write } for  pid=71727 comm=python3 name=.s.PGSQL.5432 dev="tmpfs" ino=66228 scontext=system_u:system_r:pcp_pmcd_t:s0tcontext=system_u:object_r:postgresql_var_run_t:s0 tclass=sock_file permissive=0 

* after the PMDA installation "pminfo -f postgresql" fails with an error message
postgresql: pmLookupName: No PMCD agent for domain of request

* "audit2allow -a" reports the following missing SELinux rule:
#============= pcp_pmcd_t ==============
allow pcp_pmcd_t postgresql_var_run_t:sock_file write;



Expected results:
* No AVC errors reported
* No error messages during the installation
* Collection of statistics works and "pminfo -f postgresql" shows real values.