docs: Add description of the prometheus metrics used in jormungandr | NPG-000 (#583)

# Description

Adds documentation for prometheus metrics.

## Type of change

Please delete options that are not relevant.

- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] This change requires a documentation update

## Checklist

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Oleksandr Prokhorenko <[email protected]>

Author: saibatizoku

book/src/10_prometheus_metrics/00.md

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+# Prometheus Metrics

book/src/10_prometheus_metrics/01_description.md

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+# `jormungandr` Prometheus Metrics
+
+`jormungadr` uses Prometheus metrics to gather information about the node at runtime.
+
+## Fragment Mempool Process
+
+As the node receives fragments, they are inserted into the fragment mempool, and propagated into the peer network.
+
+### `txRecvCnt`
+
+    >> tx_recv_cnt: IntCounter,
+
+Total number of tx inserted and propagated by the mempool at each loop in the process.
+
+### `txRejectedCnt`
+
+    >> tx_rejected_cnt: IntCounter,
+
+Total number of tx rejected by the mempool at each loop in the process.
+
+### `mempoolTxCount`
+
+    >> mempool_tx_count: UIntGauge,
+
+Total number of tx in the mempool for a given block
+
+### `mempoolUsageRatio`
+
+    >> mempool_usage_ratio: Gauge,
+
+Mempool usage ratio for a given block
+
+## Topology Process
+
+As the node connects to peers, the network topology allows for gossip and p2p communication. Nodes can join or leave the network.
+
+### `peerConnectedCnt`
+
+    >> peer_connected_cnt: UIntGauge,
+
+The total number of connected peers.
+
+### `peerQuarantinedCnt`
+
+    >> peer_quarantined_cnt: UIntGauge,
+
+The total number of quarantined peers.
+
+### `peerAvailableCnt`
+
+    >> peer_available_cnt: UIntGauge,
+
+The total number of available peers.
+
+### `peerTotalCnt`
+
+    >> peer_total_cnt: UIntGauge,
+
+The total number of peers.
+
+## Blockchain Process
+
+Each node receives blocks streamed from the network which are processed in order to create a new block tip.
+
+### `blockRecvCnt`
+
+    >> block_recv_cnt: IntCounter,
+
+This is the total number of blocks streamed from the network that will be processed at each loop in the process.
+
+## Blockchain Tip-Block Process
+
+As the node sets the tip-block, this happens when the node is started and during the block minting process, these metrics are updated.
+
+### `votesCasted`
+
+    >> votes_casted_cnt: IntCounter,
+
+The total number accepted `VoteCast` fragments. Metric is incremented by the total number of valid `VoteCast` fragments
+in the block tip.
+
+### `lastBlockTx`
+
+    >> // Total number of tx for a given block
+    >> block_tx_count: IntCounter,
+
+The total number of valid transaction fragments in the block tip.
+
+### `lastBlockInputTime` <--- **misnomer**
+
+    >> block_input_sum: UIntGauge,
+
+The total sum of transaction input values in the block tip. The `tx.total_input()` is added for every fragment.
+
+### `lastBlockSum`
+
+    >> block_fee_sum: UIntGauge,
+
+The total sum of transaction output values (fees) in the block tip. The `tx.total_output()` is added for every fragment.
+
+### `lastBlockContentSize`
+
+    >> block_content_size: UIntGauge,
+
+The total size in bytes of the sum of the transaction content in the block tip.
+
+### `lastBlockEpoch`
+
+    >> block_epoch: UIntGauge,
+
+The epoch of the block date defined in the block tip header.
+
+### `lastBlockSlot`
+
+    >> block_slot: UIntGauge,
+
+The slot of the block date defined in the block tip header.
+
+### `lastBlockHeight`
+
+    >> block_chain_length: UIntGauge,
+
+Length of the blockchain.
+
+### `lastBlockDate`
+
+    >> block_time: UIntGauge,
+
+Timestamp in seconds of the block date.
+
+## Unused metrics
+
+### `lastReceivedBlockTime`
+
+    >> slot_start_time: UIntGauge,
+
+This metric is never updated.
+
+## Unclear metrics
+
+### `lastBlockHashPiece`
+
+    >> block_hash: Vec<UIntGauge>,
+
+A vector of gauges that does something to with the block hash. Metric is updated when `http_response` is called.