test: Use eventually to avoid flakey test

[cbuto]

This test has been a bit flakey specifically when running in github actions.

Since the adding charts to the index is async, sometimes the metrics endpoint in this tests was being called before the second chart version was added to the index and thus the metrics were not updated.

example failure: https://github.com/helm/chartmuseum/actions/runs/5287153415/jobs/9567311265#step:5:835

2023-06-16T07:03:27.582Z	DEBUG	[33] Incoming request: /api/a/charts	{"reqID": "ab32022d-e8ee-47c9-b984-05a3d887b416"}
2023-06-16T07:03:27.583Z	DEBUG	[33] Adding package to storage	{"package": "mychart-0.1.0.tgz", "reqID": "ab32022d-e8ee-47c9-b984-05a3d887b416"}
2023-06-16T07:03:27.583Z	DEBUG	[33] PutWithLimit: per-chart-limit not set	{"reqID": "ab32022d-e8ee-47c9-b984-05a3d887b416"}
2023-06-16T07:03:27.583Z	DEBUG	[26] Entry saved in cache store	{"repo": "", "reqID": "5599daba-a3da-47fb-8915-a187d1def7ae"}
2023-06-16T07:03:27.583Z	DEBUG	[26] Event handled successfully	{"event": {"repo_name":"","operation_type":0,"chart_version":{"name":"mychart","version":"0.1.0","apiVersion":"v1","urls":["charts/mychart-0.1.0.tgz"],"created":"2023-06-16T07:03:27.577265453Z","digest":"14cd9996cc8eb0a3ef913ade40a3b000ab3dd3f5606595747cdd93786cc25d1f"}}, "reqID": "5599daba-a3da-47fb-8915-a187d1def7ae"}
2023-06-16T07:03:27.584Z	INFO	[33] Request served	{"path": "/api/a/charts", "comment": "", "clientIP": "", "method": "POST", "statusCode": 201, "latency": "2.140022ms", "reqID": "ab32022d-e8ee-47c9-b984-05a3d887b416"}
2023-06-16T07:03:27.584Z	DEBUG	[34] Incoming request: /api/a/charts	{"reqID": "436294b7-115e-44b0-bd4c-1d9b5feb38b9"}
2023-06-16T07:03:27.585Z	DEBUG	[33] Event received	{"event": {"repo_name":"a","operation_type":1,"chart_version":{"name":"mychart","version":"0.1.0","apiVersion":"v1","urls":["charts/mychart-0.1.0.tgz"],"created":"2023-06-16T07:03:27.5[836](https://github.com/helm/chartmuseum/actions/runs/5287153415/jobs/9567311265#step:5:837)95319Z","digest":"14cd9996cc8eb0a3ef913ade40a3b000ab3dd3f5606595747cdd93786cc25d1f"}}, "reqID": "ab32022d-e8ee-47c9-b984-05a3d887b416"}
2023-06-16T07:03:27.585Z	DEBUG	[34] Adding package to storage	{"package": "mychart-0.2.0.tgz", "reqID": "436294b7-115e-44b0-bd4c-1d9b5feb38b9"}
2023-06-16T07:03:27.585Z	DEBUG	[34] PutWithLimit: per-chart-limit not set	{"reqID": "436294b7-115e-44b0-bd4c-1d9b5feb38b9"}
2023-06-16T07:03:27.587Z	DEBUG	[33] Entry saved in cache store	{"repo": "a", "reqID": "ab32022d-e8ee-47c9-b984-05a3d887b416"}
2023-06-16T07:03:27.587Z	DEBUG	[33] Event handled successfully	{"event": {"repo_name":"a","operation_type":1,"chart_version":{"name":"mychart","version":"0.1.0","apiVersion":"v1","urls":["charts/mychart-0.1.0.tgz"],"created":"2023-06-16T07:03:27.583695319Z","digest":"14cd9996cc8eb0a3ef913ade40a3b000ab3dd3f5606595747cdd93786cc25d1f"}}, "reqID": "ab32022d-e8ee-47c9-b984-05a3d887b416"}
2023-06-16T07:03:27.587Z	INFO	[34] Request served	{"path": "/api/a/charts", "comment": "", "clientIP": "", "method": "POST", "statusCode": 201, "latency": "2.92633ms", "reqID": "436294b7-115e-44b0-bd4c-1d9b5feb38b9"}
2023-06-16T07:03:27.587Z	DEBUG	[35] Incoming request: /a/index.yaml	{"reqID": "d9679b71-d703-4aed-860d-0c06f31cc94b"}
2023-06-16T07:03:27.587Z	DEBUG	[35] Entry found in cache store	{"repo": "a", "reqID": "d9679b71-d703-4aed-860d-0c06f31cc94b"}
2023-06-16T07:03:27.588Z	INFO	[35] Request served	{"path": "/a/index.yaml", "comment": "", "clientIP": "", "method": "GET", "statusCode": 200, "latency": "165.602µs", "reqID": "d9679b71-d703-4aed-860d-0c06f31cc94b"}
2023-06-16T07:03:27.588Z	DEBUG	[36] Incoming request: /b/index.yaml	{"reqID": "002aca9b-5f06-400d-9254-fb4cdbcb69cb"}
2023-06-16T07:03:27.588Z	DEBUG	[36] Fetching chart list from storage	{"repo": "b", "reqID": "002aca9b-5f06-400d-9254-fb4cdbcb69cb"}
2023-06-16T07:03:27.588Z	DEBUG	[36] No change detected between cache and storage	{"repo": "b", "reqID": "002aca9b-5f06-400d-9254-fb4cdbcb69cb"}
2023-06-16T07:03:27.589Z	INFO	[36] Request served	{"path": "/b/index.yaml", "comment": "", "clientIP": "", "method": "GET", "statusCode": 200, "latency": "766.308µs", "reqID": "002aca9b-5f06-400d-9254-fb4cdbcb69cb"}
2023-06-16T07:03:27.589Z	DEBUG	[37] Incoming request: /metrics	{"reqID": "be507d15-2fb8-4493-b24f-08fe641aa321"}

# second chart version event was just received (not successfully processed yet)
2023-06-16T07:03:27.591Z	DEBUG	[34] Event received	{"event": {"repo_name":"a","operation_type":1,"chart_version":{"name":"mychart","version":"0.2.0","apiVersion":"v1","urls":["charts/mychart-0.2.0.tgz"],"created":"2023-06-16T07:03:27.585890[841](https://github.com/helm/chartmuseum/actions/runs/5287153415/jobs/9567311265#step:5:842)Z","digest":"5f2f1145e113d6c39260ef4be82e96f86e352eae156db006105d366b5d354cb8"}}, "reqID": "436294b7-115e-44b0-bd4c-1d9b5feb38b9"}
2023-06-16T07:03:27.591Z	DEBUG	[34] Entry found in cache store	{"repo": "a", "reqID": "436294b7-115e-44b0-bd4c-1d9b5feb38b9"}

# metrics endpoint is called
2023-06-16T07:03:27.592Z	INFO	[37] Request served	{"path": "/metrics", "comment": "", "clientIP": "", "method": "GET", "statusCode": 200, "latency": "3.426435ms", "reqID": "be507d15-2fb8-4493-b24f-08fe641aa321"}
    server_test.go:1008: 
        	Error Trace:	/home/runner/work/chartmuseum/chartmuseum/pkg/chartmuseum/server/multitenant/server_test.go:1008
        	Error:      	Should be true
        	Test:       	TestMultiTenantServerTestSuite/TestMetrics