feat: Add granted label pg_locks collector

[eugercek]

Hi,

This PR adds granted label to pg_locks_count metric.

Implements and Closes #657.

Implementation Choices:

Postgres version compatibility: I believe pg_locks.granted exists since postgres started to using Git. This is the commit that src/backend/catalog/system_views.sql file is added and pg_locks.granted exists at that time too. Nov 11 2003. Also in 7.3.0 release notes pg_locks is mentioned. So It'll be available since 7.3.0, because of these I did not use instance.version.GE like check, it's too old.

Metric format: I did not touch current format it'll be cartesian product, value=0 will be exists. Only added granted label.

How bool is implemented: Since we already use data point's value for count of the current data point, like in below metric we know that there are 9 locks for granted="true",mode="accesssharelock", I can't use the metric's value like pg_replication_slot_slot_is_active, so used a new label called granted.

pg_locks_count{datname="postgres",granted="true",mode="accesssharelock"} 9`

Because we use count value I couldn't use this standard. Found open-telemetry/opentelemetry-dotnet#4822 standard looks like using lowercase "true" "false" so used that.

Cardinality and Metric Count: Cardinality will be 2, just granted="true|false". But metric count will be doubled because of the added 2 new dimension.

Tests

Unit Test:

go test ./collector -run TestPGLocksCollector -v
=== RUN   TestPGLocksCollector

  Metrics comparison ✔


1 total assertion

--- PASS: TestPGLocksCollector (0.00s)
PASS
ok      github.com/prometheus-community/postgres_exporter/collector     (cached)

Manual test

Ran below:

CREATE DATABASE foo;
-- \c foo
CREATE TABLE foo(name text);

begin;
LOCK TABLE foo IN EXCLUSIVE MODE; 

-- In another session
begin;
LOCK TABLE foo IN EXCLUSIVE MODE; 

curl -s http://localhost:9187/metrics | grep pg_locks | grep foo

Output

pg_locks_count{datname="foo",granted="false",mode="accessexclusivelock"} 0
pg_locks_count{datname="foo",granted="false",mode="accesssharelock"} 0
pg_locks_count{datname="foo",granted="false",mode="exclusivelock"} 1
pg_locks_count{datname="foo",granted="false",mode="rowexclusivelock"} 0
pg_locks_count{datname="foo",granted="false",mode="rowsharelock"} 0
pg_locks_count{datname="foo",granted="false",mode="sharelock"} 0
pg_locks_count{datname="foo",granted="false",mode="sharerowexclusivelock"} 0
pg_locks_count{datname="foo",granted="false",mode="shareupdateexclusivelock"} 0
pg_locks_count{datname="foo",granted="false",mode="sireadlock"} 0
pg_locks_count{datname="foo",granted="true",mode="accessexclusivelock"} 0
pg_locks_count{datname="foo",granted="true",mode="accesssharelock"} 0
pg_locks_count{datname="foo",granted="true",mode="exclusivelock"} 1
pg_locks_count{datname="foo",granted="true",mode="rowexclusivelock"} 0
pg_locks_count{datname="foo",granted="true",mode="rowsharelock"} 0
pg_locks_count{datname="foo",granted="true",mode="sharelock"} 0
pg_locks_count{datname="foo",granted="true",mode="sharerowexclusivelock"} 0
pg_locks_count{datname="foo",granted="true",mode="shareupdateexclusivelock"} 0
pg_locks_count{datname="foo",granted="true",mode="sireadlock"} 0