feat: mempool stats endpoint and prometheus metrics (#1241)

* feat: implement endpoint for various mempool stats

* chore: remove coinbase tx entries from mempool stats (will always be empty)

* chore: update prometheus mempool stats on new block, microblock, and mempool tx

* chore: add mempool tx byte size stats

* chore: add indexes to speedup mempool table queries

* chore: move `registerMempoolPromStats` fn to db helper module

* docs: add OpenAPI schema for new mempool stats endpoint

* chore: tests for mempool stats endpoint

* chore: restore mempool garbage collect env var after test

* chore: bytea serialization tests

* chore: strict hex string handling for bytea value serializer

* chore: improved mempool_tx indexing

* chore: unit test fix

Author: zone117x

docs/api/transaction/get-mempool-stats.example.json

@@ -0,0 +1,86 @@
+{
+  "tx_type_counts": {
+    "token_transfer": 130,
+    "smart_contract": 2,
+    "contract_call": 310,
+    "poison_microblock": 0
+  },
+  "tx_simple_fee_averages": {
+    "token_transfer": {
+      "p25": 3000,
+      "p50": 3000,
+      "p75": 6000,
+      "p95": 401199.9999999995
+    },
+    "smart_contract": {
+      "p25": 837500,
+      "p50": 925000,
+      "p75": 1012500,
+      "p95": 1082500
+    },
+    "contract_call": {
+      "p25": 3000,
+      "p50": 10368,
+      "p75": 100000,
+      "p95": 1000000
+    },
+    "poison_microblock": {
+      "p25": null,
+      "p50": null,
+      "p75": null,
+      "p95": null
+    }
+  },
+  "tx_ages": {
+    "token_transfer": {
+      "p25": 167.5,
+      "p50": 45,
+      "p75": 1,
+      "p95": 0
+    },
+    "smart_contract": {
+      "p25": 185.5,
+      "p50": 129,
+      "p75": 72.5,
+      "p95": 27.30000000000001
+    },
+    "contract_call": {
+      "p25": 189,
+      "p50": 127.5,
+      "p75": 9.5,
+      "p95": 0
+    },
+    "poison_microblock": {
+      "p25": null,
+      "p50": null,
+      "p75": null,
+      "p95": null
+    }
+  },
+  "tx_byte_sizes": {
+    "token_transfer": {
+      "p25": 180,
+      "p50": 180,
+      "p75": 180,
+      "p95": 180
+    },
+    "smart_contract": {
+      "p25": 706.75,
+      "p50": 814.5,
+      "p75": 922.25,
+      "p95": 1008.45
+    },
+    "contract_call": {
+      "p25": 291,
+      "p50": 435,
+      "p75": 462,
+      "p95": 597
+    },
+    "poison_microblock": {
+      "p25": null,
+      "p50": null,
+      "p75": null,
+      "p95": null
+    }
+  }
+}