doc: rdd package documentation

Author: RawanMostafa08

docs/internals/rrd/readme.md

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+# RRD Package
+
+## Introduction
+
+Package rrd provides a Round Robin Database (RRD) implementation using BoltDB.
+
+The RRD is designed to store monotonically increasing counter values, making it easy to compute the increase of a metric over a fixed time window. Data is stored in fixed-size time slots, and older data is automatically deleted after the retention duration. 
+
+## Structure
+
+**Interfaces:**                 
+   - `RRD` : the main API for interacting with the DB 
+   - `Slot` : represents one time window bucket      
+   - `Printer` : for printing DB contents             
+                             
+**Structs:**              
+   - `rrdBolt` implements `RRD`
+   - `rrdSlot` implements `Slot`
+
+### 1- RRD Interface
+```go
+type RRD interface {
+	// Slot returns the current window (slot) to store values.
+	Slot() (Slot, error)
+
+	// Counters, return all stored counters since the given time (since) until now.
+	Counters(since time.Time) (map[string]float64, error)
+
+	// Last returns the last reported value for a metric given the metric name
+	Last(key string) (value float64, ok bool, err error)
+
+	// Close the db
+	Close() error
+}
+```
+
+#### Constructor
+
+```go
+// NewRRDBolt creates a new rrd database that uses bolt as a storage
+// if window or retention are 0, the function will panic.
+// If retnetion is smaller then window the function will panic.
+// retention and window must be multiple of 1 minute.
+func NewRRDBolt(path string, window time.Duration, retention time.Duration) (RRD, error)
+```
+
+#### rrdBolt Struct
+`rrdBolt` implements `RRD` and also implements the `Printer` interface
+
+```go
+type rrdBolt struct { 
+	db        *bolt.DB // BoltDB database handle
+	window    uint64   // Window size in seconds
+	retention uint64   // Retention duration in seconds
+}
+
+// RDD interface methods
+Slot() (Slot, error)
+Counters(since time.Time) (map[string]float64, error)
+Last(key string) (value float64, ok bool, err error)
+Close() error
+
+// Printer interface methods 
+Print(out io.Writer) error
+
+// extra methods
+// Slots return all the slot timestamps stored in database
+Slots() ([]uint64, error)
+```
+
+### 2- Slot Interface
+
+```go
+type Slot interface {
+	// Counter sets (or overrides) the current stored value for this key, with the passed value
+	Counter(key string, value float64) error
+
+	// Key return the key of the slot which is the window timestamp
+	Key() uint64
+}
+```
+
+#### rrdSlot Struct
+`rrdSlot` implements `Slot`
+
+```go
+type rrdSlot struct {
+	db  *bolt.DB // BoltDB database handle
+	key uint64   // slot timestamp
+}
+
+// Slot interface methods
+Counter(key string, value float64) error
+Key() uint64
+```
+
+### 3- Printer Interface
+
+```go
+type Printer interface {
+    // Print writes the content of the RRD to the passed writer
+	Print(w io.Writer) error
+}
+```