Add SigV4 Support

[lewinkedrs]

Issue #745 . This pull requests adds SigV4 support to Promxy for Querying of Amazon Managed Prometheus workspaces. All tests are passing and I confirmed i can query 2 AMP workspaces in a federated query with Promski.

{
    "status": "success",
    "data": {
        "resultType": "vector",
        "result": [
            {
                "metric": {
                    "__name__": "up",
                    "workspace": "workspace-1",
                    ...
                },
                "value": [1771607847.645, "1"]
            },
            {
                "metric": {
                    "__name__": "up",
                    "workspace": "workspace-2",
                    ...
                },
                "value": [1771607847.645, "1"]
            }
        ]
    }
}

Package Test Summary

Package	Status	Duration
pkg/alertbackfill	✅ PASS	cached
pkg/config	✅ PASS	cached
pkg/logging	✅ PASS	cached
pkg/middleware	✅ PASS	cached
pkg/promclient	✅ PASS	cached
pkg/promhttputil	✅ PASS	cached
pkg/proxyquerier	✅ PASS	no test files
pkg/proxystorage	✅ PASS	cached
pkg/remote	✅ PASS	cached
pkg/server	✅ PASS	cached
pkg/servergroup	✅ PASS	cached
test	✅ PASS	cached

[lewinkedrs]

Any feedback on this?

[jacksontj]

Thanks for the contribution! I have kicked off the CI run; assuming this looks good we can merge this in!

[pedroapero]

We are using aws-sigv4-proxy sidecar containers but your way is definitely better (might require more code maintenance though).

[lewinkedrs]

I pushed a change to not require aws provider chain to be set for test to pass. I think we should pass the CI build now.