Compiling version of the postgres_exporter.

This is based on the mysqld_exporter, but drastically changes how metrics are
discovered and enumerated since postgres already very closely follows the
Prometheus format.

README.md

@@ -0,0 +1,38 @@
+# PostgresSQL Server Exporter
+
+Prometheus exporter for PostgresSQL server metrics.
+Supported Postgres versions: 9.1 and up.
+
+## Building and running
+
+    make
+    export DATA_SOURCE_NAME="login:password@(hostname:port)/dbname"
+    ./postgres_exporter <flags>
+
+### Flags
+
+Name               | Description
+-------------------|------------
+web.listen-address | Address to listen on for web interface and telemetry.
+web.telemetry-path | Path under which to expose metrics.
+
+### Setting the Postgres server's data source name
+
+The PostgresSQL server's [data source name](http://en.wikipedia.org/wiki/Data_source_name)
+must be set via the `DATA_SOURCE_NAME` environment variable.
+The format of this variable is described at https://github.com/go-sql-driver/mysql#dsn-data-source-name.
+
+### Adding new metrics
+
+The exporter will attempt to dynamically export additional metrics if they are added in the
+future, but they will be marked as "untyped". Additional metric maps can be easily created
+from Postgres documentation by copying the tables and using the following Python snippet:
+
+```python
+x = """tab separated raw text of a documentation table"""
+for l in StringIO(x):
+    column, ctype, description = l.split('\t')
+    print """"{0}" : {{ prometheus.CounterValue, prometheus.NewDesc("pg_stat_database_{0}", "{2}", nil, nil) }}, """.format(column.strip(), ctype, description.strip())
+```
+Adjust the value of the resultant prometheus value type appropriately. This helps build
+rich self-documenting metrics for the exporter.