Add support for custom transfers metrics

[stefangutica]

Reasoning

• custom transfers subscription doesn't appear in grafana metrics

Proposed Changes

• add support for custom transfers metrics

How to test

• should appear on grafana

[github-actions]

k6 load testing comparison.
Base Commit Hash: 8aa0423
Target Commit Hash: dc412af

Metric	Base				Target				Diff
	Avg	Max	90	95	Avg	Max	90	95	Avg	Max	90	95
Tokens	59.98	463.20	64.17	175.77	52.11	1280.16	54.31	56.54	-13.13% ✅	+176.37% 🔴	-15.38% ✅	-67.83% ✅
Mex	60.39	514.00	64.14	178.54	51.64	860.31	54.43	56.81	-14.49% ✅	+67.38% 🔴	-15.15% ✅	-68.18% ✅
Accounts	72.67	549.38	108.19	307.23	102.03	244.08	108.14	116.94	+40.40% 🔴	-55.57% ✅	-0.05% ✅	-61.94% ✅
Blocks	67.95	730.15	64.94	294.98	72.68	516.85	83.33	87.46	+6.95% 🔴	-29.21% ✅	+28.32% 🔴	-70.35% ✅
Transactions	164.24	22099.88	125.21	476.89	102.58	270.27	107.24	115.33	-37.54% ✅	-98.78% ✅	-14.36% ✅	-75.82% ✅
Nodes	150.41	19411.62	281.43	306.52	51.49	537.01	54.41	56.74	-65.77% ✅	-97.23% ✅	-80.66% ✅	-81.49% ✅
Pool	59.77	462.07	64.10	154.41	51.89	828.68	54.38	56.79	-13.19% ✅	+79.34% 🔴	-15.17% ✅	-63.22% ✅
Test Run Duration	60004.79				60002.76

Legend: Avg - Average Response Time, Max - Maximum Response Time, 90 - 90th Percentile, 95 - 95th Percentile
All times are in milliseconds.