Moves examples/local-otel-collector to examples/with-otel and transforms
it into a proper Turborepo example following the create-turbo pattern.

- Full monorepo structure (apps/web, apps/docs, shared packages)
- Installed turbo@2.8.13-canary.1 as a devDependency
- Pre-configured Grafana with a Turborepo Runs dashboard
- Anonymous Grafana access so users see metrics immediately
- meta.json, pnpm-workspace.yaml, and all standard boilerplate

…rimentalObservability: true`, causing the OTEL example to fail with an error when following the documented instructions.

This commit fixes the issue reported at examples/with-otel/turbo.json:2

## Bug Analysis

The `examples/with-otel` example documents a workflow where users set environment variables like `TURBO_EXPERIMENTAL_OTEL_ENABLED=1` and then run `turbo build` to export OpenTelemetry metrics. However, the example's `turbo.json` is missing the required `futureFlags.experimentalObservability: true` setting.

### How the gate works

In `crates/turborepo-lib/src/run/builder.rs` (lines 453-463), there is a hard gate:

```rust
if let Some(obs_opts) = &self.opts.experimental_observability {
    if obs_opts.otel.is_some() && !self.opts.future_flags.experimental_observability {
        return Err(turborepo_config::Error::InvalidExperimentalOtelConfig {
            message: "experimentalObservability.otel is configured but \
                      futureFlags.experimentalObservability is not enabled in turbo.json."
                .to_string(),
        }
        .into());
    }
}
```

### The flow

1. User sets `TURBO_EXPERIMENTAL_OTEL_ENABLED=1` (and other env vars) as documented in the README
2. The env vars are parsed in `crates/turborepo-config/src/experimental_otel.rs` into `ExperimentalOtelOptions`
3. This gets wrapped as `ExperimentalObservabilityOptions { otel: Some(otel) }` in the config funnel
4. When `turbo build` runs, the gate checks `obs_opts.otel.is_some()` (true) AND `!self.opts.future_flags.experimental_observability` (true, since `FutureFlags` defaults to `false`)
5. The run fails with the error: "experimentalObservability.otel is configured but futureFlags.experimentalObservability is not enabled in turbo.json."

### The fix

Added the `futureFlags` block with `experimentalObservability: true` to `examples/with-otel/turbo.json`. This is a valid top-level field in turbo.json (defined in `crates/turborepo-turbo-json/src/raw.rs` line 291 and extensively tested). Without this, the example is completely non-functional as documented.

Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com>
Co-authored-by: anthonyshew <anthonyshew@gmail.com>

- All stat panels use sum() to aggregate across runs (no duplicate numbers)
- Cache Hit Rate shows a single gauge
- Run Duration uses instant query that works with one-off runs
- Replaced Runs Over Time with a Total Runs stat + Run Duration Over Time chart
- Task Details table uses filterFieldsByName to exclude Prometheus-injected columns
- All task breakdown queries use sum by (turbo_task_name) for clean grouping
- Time-series panels use showPoints: always + spanNulls for sparse data

…on chart