chore: disable plugin server posthog capture if not set

[daibhin]

Problem

The plugin server captures events from dev

Changes

• Restrict to only capture if the client credentials exist
• Move client credentials to be an env var (related to https://github.com/PostHog/charts/pull/3507)

[greptile-apps]

PR Summary

This PR modifies the plugin server to conditionally capture PostHog analytics based on environment variables rather than using hardcoded values.

• Changed PostHog client initialization in utils/posthog.ts to only create a client when POSTHOG_API_KEY is provided
• Added null checks before using the PostHog client in captureTeamEvent and captureException functions
• Added new environment variables POSTHOG_API_KEY and POSTHOG_HOST_URL to the PluginsServerConfig interface
• Removed previous random sampling logic (10% of exceptions) that was used in production
• Set default values in config.ts: empty string for API key and 'http://localhost:8010' for host URL

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[oliverb123]

One thing that might be nicer here, from the dev/hobby perspective, is to hit the DB and fetch team-1's token if none is set (maybe putting that behaviour behind an env var) - wdyt?

something like if (POSTHOG_API_KEY == '' && POSTHOG_AUTOCONFIGURE) { POSTHOG_API_KEY = fetchKey() }

[daibhin]

@oliverb123 thought something like this might work when creating the config:

    if (!newConfig.POSTHOG_API_KEY && newConfig.POSTHOG_AUTOCONFIGURE) {
        newConfig.POSTHOG_API_KEY = await fetchDevTeamApiToken(newConfig)
    }

    if (!Object.keys(KAFKAJS_LOG_LEVEL_MAPPING).includes(newConfig.KAFKAJS_LOG_LEVEL)) {
        throw Error(
            `Invalid KAFKAJS_LOG_LEVEL ${newConfig.KAFKAJS_LOG_LEVEL}. Valid: ${Object.keys(
                KAFKAJS_LOG_LEVEL_MAPPING
            ).join(', ')}`
        )
    }
    return newConfig
}

async function fetchDevTeamApiToken(config: PluginsServerConfig): Promise<string> {
    const postgres = new PostgresRouter(config)
    const teamManager = new TeamManager(postgres)

    const team = await teamManager.fetchTeam(1)

    if (!team) {
        throw Error('Could not find team with ID 1 to autoconfigure API token')
    }

    return team.api_token
}

Problem is that the lookup will cause things to be async which has a whole array of changes.

Given people were never getting plugin server events locally before (they were sent to production) I think it's ok to not capture them but default and let people add their keys locally if they want to test something