[+] add PlantUML diagrams (#926)

Author: pashagolub

docs/uml/.styles.puml

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+@startuml .styles
+!theme plain
+' Set background and font color
+skinparam backgroundColor #f4f7fa
+skinparam defaultTextAlignment center
+skinparam shadowing false
+skinparam titleFontColor #1c4063
+skinparam titleFontSize 20
+skinparam titleFontName Arial
+skinparam noteBackgroundColor #eaf0f6
+skinparam noteBorderColor #1c4063
+
+' General border and font color
+skinparam componentBorderColor #1c4063
+skinparam componentFontColor #222222
+skinparam componentBackgroundColor #eaf0f6
+skinparam databaseBorderColor #1c4063
+skinparam databaseFontColor #222222
+skinparam databaseBackgroundColor #dbe9f6
+skinparam packageBorderColor #1c4063
+skinparam packageTitleFontColor #1c4063
+skinparam packageBackgroundColor #eaf0f6
+
+skinparam ArrowColor #467fcf
+skinparam ArrowFontColor #1c4063
+
+skinparam packageStyle rectangle
+skinparam componentStyle rectangle
+skinparam databaseStyle rectangle
+skinparam direction top to bottom
+skinparam minClassWidth 200
+skinparam wrapWidth 200
+@enduml

docs/uml/patroni-prometheus.puml

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+@startuml patroni-prometheus
+
+!include .styles.puml
+
+title pgwatch with Prometheus - Direct patroni nodes conninfo
+
+!include patroni.puml
+
+package "pgwatch related"{
+
+    node pgwatch [
+        **pgwatch**
+        ----
+        --config=postgresql://[email protected]/pgwatch
+        --sink=prometheus://:9187/pgwatch
+    ]
+
+    database pgwatch_config_db [
+        Postgres Config DB
+        ----
+        - Monitored nodes settings
+        - Metric Definitions
+        ----
+        IP: 10.0.0.42
+        Database: pgwatch
+    ]
+
+
+    pgwatch -up-> postgres1: fetch metric measurements
+    pgwatch -up-> postgres2: fetch metric measurements
+    pgwatch -up-> postgres3: fetch metric measurements
+
+    pgwatch ..left..> pgwatch_config_db: reads metrics and monitored sources
+
+}
+
+    component Grafana
+
+    component Prometheus
+
+    Prometheus -up=> pgwatch: scrapes new measurements
+    Grafana -up=> Prometheus: reads data and shows graphs
+
+@enduml

docs/uml/patroni.puml

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+@startuml patroni
+
+!include .styles.puml
+
+package "patroni cluster"{
+
+    node etcd [
+        **etcd**
+        ----
+        Key-Value Store
+    ]
+
+    database postgres1 [
+        **postgres1**
+        ----
+        primary
+    ]
+
+    database postgres2 [
+        **postgres2**
+        ----
+        standby
+    ]
+
+    database postgres3 [
+        **postgres3**
+        ----
+        standby
+    ]
+
+
+    postgres1 -up-> etcd
+    postgres2 -up-> etcd
+    postgres3 -up-> etcd
+
+}
+@enduml