UI: use prometheus post api

[MichaHoffmann]

Downstream Requests to the query API will fail for long queries if there is a proxy in the middle that fails on too long URIs.

• [ ] I added CHANGELOG entry for this change.
• [ ] Change is not relevant to the end user.

Changes

Make the UI use POST requests to the query api.

Verification

[MichaHoffmann]

cc @kevindweb

[kevindweb]

For context the use case is dynamically injecting label filters into client metrics. When the query has 10+ series, each with a large set of generated filters, using a POST succeeds on the backend, but our clients cannot visualize the query in the UI due to size constraints.