fix: change MetricsPusher to use import prometheus endpoint

[edgurgel]

What kind of change does this PR introduce?

No need to use snappy and rust. We can simply send the prometheus export format

More info: https://docs.victoriametrics.com/victoriametrics/single-server-victoriametrics/#how-to-import-data-in-prometheus-exposition-format & https://docs.victoriametrics.com/victoriametrics/single-server-victoriametrics/#how-to-import-time-series-data

[Copilot]

Pull request overview

This PR migrates MetricsPusher from Prometheus remote_write (protobuf + snappy via a Rustler NIF) to pushing plain Prometheus exposition text to a Prometheus import endpoint, optionally gzip-compressed.

Changes:

• Switch MetricsPusher request format/headers to text/plain and use Req gzip body compression (compress_body) instead of remote_write encoding.
• Remove the Rustler-based PrometheusRemoteWrite implementation and its associated Rust/native build/tooling.
• Update MetricsPusher tests and docs to reflect the new import endpoint and gzip option.

Reviewed changes

Copilot reviewed 14 out of 16 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
lib/realtime/metrics_pusher.ex	Send Prometheus text format directly, optionally gzip-compressed, and adjust headers/auth setup.
test/realtime/metrics_pusher_test.exs	Update expectations to import endpoint + gzip, add a compress=false case, remove remote_write decoding assertions.
config/test.exs	Sets a default metrics_pusher_auth for tests.
README.md	Clarify that METRICS_PUSHER_URL is for Prometheus exposition format import.
mix.exs	Remove :rustler dependency.
Dockerfile	Remove Rust toolchain install and native crate copy step.
.gitignore	Remove Rust/Rustler build artifact ignores.
Removed files (native/prometheus_remote_write/**, tests/fixtures)	Delete the remote_write NIF implementation and its tests/fixtures and workflow.

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[coveralls]

coverage: 92.171% (+0.009%) from 92.162%
when pulling 347e64d on fix/metrics-pusher
into 5816459 on main.